
My dog showed similar symptoms georgepig, and they were on/off for many months - sometimes the symptoms could disappear for weeks at a time and then the next day we could seem to be right back at square one.
I think it was March last year it started. One day after having a rest following a walk he was very stiff in the leg, held it for a few seconds and then seemed completely normal. He's had a few injuries before (he's a total loon for sprinting after squirrels and rabbits) so I just did the usual and gave him some anti-inflammatories (okayed with the vets) and kept him on the lead for a week. He then seemed fine for several weeks once let back off, but the leg then went again. Kept him on the lead for 3 weeks this time and he was then fine for pretty much the whole summer. Then the leg went yet again! The vets then advised a much longer bout of rest and lead walks than we had done so far as a final last ditch attempt before further investigations. We did 10 weeks this time which took us up to last December. He'd been back off the lead for 2 weeks, had chased a few squirrels and everything seemed fine. Then one day he did a particular hard run at a squirrel in the distance - the squirrel ran across his path and then did a sharp turn followed by my dog doing a 180 degree turn on the spot at top speed. He then stood there and held the leg high in the air for about 3 seconds before setting off after another one!

He walked home completely normal but after resting seemed to have no strength in his back end at all and had to first pull himself up into a sit before he was able to haul his back end up.
He was then admitted for investigations and referred to a specialist. He has a tear not a total rupture hence the on/off symptoms and the ability to still walk and even run on it without apparent problem (other than lifting the leg for a few seconds after the squirrel sprint, the only time you ever really "saw" the injury was when rising after rest (I was told that's because the swelling/fluid in the knee takes time to build up after trauma to the knee which is why the dog settles down to sleep completely normal and then wakes up with a problem).
We saw the specialist in January (2 weeks after being back on the lead again) and he couldn't detect anything wrong with his movement at all (the xrays showed inflammation and fluid, though this was undetectable to the touch). He advised us that there is a small chance that the knee could restabilise itself with scar tissue but that it would take many months of rest and restriction. Seeing that he improves so rapidly after each reinjury this is the option that he advised us to take. It is very hard as I have to keep him on lead in the garden and even in the house as he dashes to the door to bark at the postman, newspaper boy etc or the windows if he thinks he hears/sees something. We're over 3 months in now and up to 20 minute lead walks (he could quite happily go for miles). For all intents and purposes he is completely fine, but I certainly won't be letting him off lead for at least another 3 months. We do hydrotherapy lined up but haven't started that yet.
Surgery is definitely not the only option. I think there are 3 or 4 members here who have got their dog over a cruciate ligament injury without surgery and I know of an excellent website by someone whos dog totally ruptured both ligaments and also opted not to have surgery (5 years on the dog has not had another problem and shows no signs of arthritis despite being told that severe arthritis will set in without surgery). I can get the web address if you are interested.