
Hi Spike,
she may be having an allergic reaction to something in the environment that she walks, plays, sleeps on etc either indoor or outdoor (or both I suppose). Do you perhaps use alternating varieties of cleaning materials on carpets, wood floors, tiles etc or in your laundry if washing her bedding? This could be a reason why her itching is sporadic. Alternatively is there a link between flare ups and where you exercise her - it may be that certain plants, grasses or chemicals used on fields / outdoor surfaces are linked?
Ideally you should have your vet examine her and perhaps take a skin scraping but if you could try and establish any possible patterns in your domestic set up that coincide with these periods it will help to identify or rule things out for when discussing with your vet.
HTH, best wishes Teri :)