Right - the chances are this is NOT Cocker Rage Syndrome - Rage syndrome is classically displayed in a sudden switch to EXTREMELY aggressive, violent behaviour with NO obvious or even less obvious trigger. Your dog quite clearly has a very predictable trigger - resource guarding.
I have to say, its more than likely you did not buy her from a reputable, responsible breeder and actually fell into the trap a lot of puppy farmers and disreputable breeders create - not your fault but you know now.
Back to your dog - it is likely that the resource guarding has become worse because you have been telling her off. That might sound crazy, but when she has something, it IS hers. You will no more teach her that it isn't than you will teach her to drive a car or read Latin. Do not bother.
She resource guards because you want to 'steal her stuff' - she has no idea that its your stuff (its in her mouth or by her paws, its hers) nor that it is dangerous or expensive or you don't want her to have it for other really good reasons. All she knows and understands is this:
"I have something. They will take it. Even if I make it clear with my body language and noises, they will STILL try to take it, so to keep my stuff I must use aggressive behaviour'.
The more you tell her off and try to take things from her, the worse she will get.
So - first of all hold off spaying - its not going to help this problem, and it may make it worse.
Next - tidy your house until it is like some minimalistic show home - if that means hiding things in random drawers and cupboards, do it - do not leave ANYTHING she might take within reach.
Next step - if she does get hold of something, no matter what it is, I don't care if its a 12" carving knife, a stick of Acme Dynamite or the Holy Grail...
you must say NOTHING and you must WALK AWAY.
Go into another room, make noises like you are getting food or picking your keys up or picking up a dog lead to go out, or you have a squeaky toy - anything that makes her think 'hey? I got NO attention from this AND... they are doing something I'd like to do in there...'.
Then follow through with that promise, so if you jangled the keys then you take her out. If you picked up the leash, you take her out. If you rattled some food, you give her a treat. Never lie to a dog, they won't let you do it very often! You don't have to take her for a long walk or give her a huge fist full of treats, just sufficient for her to be satisfied, and then YOU are back in control of the situation, you can put her in another room and remove whatever it was she had.
You again probably think this is nuts, and that it is either not dealing with the issue or it is even rewarding her for it - it isn't, please believe me. If she DID have a 12" carving knife in her chops and you confront her, whats going to happen? Calmly handing over the knife, no damage done? Or mental crazed dog running about the house snarling, biting and wielding or worse, tryign to swallow, a sharp knife?.. The same applies to practically every situation you could find yourselves in here - a dog with a dangerous or valuable object is MORE dangerous when you confront her, threaten her and she rushes about defending her goodies or trying to 'get rid' by swallowing. Even if you DO manage to wrestle the object off her, you just teach her to be more dangerous and aggressive and quick to react the next time.
You are also NOT rewarding her for the behaviour - by leaving the room and luring her out to do something else you are changing the subject, once shes dropped the item and come to see what you are doing, the 'she stole something' situation is over, finished, dealt with. As I said before, since you won't ever be able to teach her that taking things is 'wrong', there is no point berating her after the fact and it will just make things worse (it already has!).
This however is just a management strategy - this is not the cure in and of itself.
To address that, once you have the management in place, ie, you are preventing her from doign it as much as possible, when it DOES happen you know what to do to keep the situation safe and not make it any worse... you need to teach her that giving you stuff and leaving stuff is VERY rewarding for her to do.
This is as much habit as it is training - thats why it is crucial that you take steps to get rid of the old habit first. It will die, IF you let it - if you constantly challenge her for things and take things from her forcibly then the habit of suspecting you will do this (and being right) and then needing to defend things will remain.
What you do is you teach her, starting with REALLY low value items (ie, nothing shes ever taken and guarded, things she is already happy to hand over) you teach her to swap.
You start out with swapping the low value item for a really high value item, that is right there in your hand, she can see it. In other words, outright bribery.
Thats a piece of cake - who wants a boring thing when they could 'drop it' or 'give it' and earn a TASTY thing! Easy peasy.
Do that in every room of your house - dogs do not generalise well you have to train in LOTS of locations before they will generalise.
When she can give/drop/leave an item in every room in your house, for the high value reward you have practically under her nose, then you make the game a little harder.
You can EITHER, make the item a bit more interesting, OR make the treat a bit less bribey (ie, hold it in a closed fist, further from her). Pick whichever you think will be easiest for her to figure out.
When you can do the exercise in all rooms with that change in place, introduce another change, again make the item a touch more interesting OR make the reward a bit less bribey.
It is SUPER important that you do NOT change both the value of the item she is trading AND the value/level of bribery of the treat she is trading FOR at the same time - change one variable at a time only. When you get to super hard items you may even need to drop back down a few levels on the other aspects - for example say in six months you are working on her swapping you a raw steak... and shes been working for kibble hidden in another room, it would be advisable to go back to using warm stinky cheese under her nose.
Anyway I think thats enough info for now - you will get brain-overload otherwise. The key points right now are DO NOT CHALLENGE HER, and to manage her so she cannot do this.