i was hoping to get a response back but don't want you to go without an answer either. Often a dog with a soft tissue injury will start to heal only to reinjure themselves leading to the limps return. That is one possible scenario. Some smaller dogs have luxating patellas. A luxating patella is when the knee pops in and out of joint. The more often it pops out, the easier it will pop out in the future. Often a dog will limp but extending the leg all the way out will help it pop back into place allowing them to run or climb stairs etc. The following explains this:
The following site goes over the surgery to correct this.
Surgery that is performed early has a higher chance of success as the more the knee pops in and out of the joint, the easier it is for it to do so.
Lyme disease is another possible cause for a limp that comes and goes especially if it switches legs.
The primary symptom of Lyme disease is arthritis 2 to 5 months after tick exposure and generally strikes the joints closest to the tick bites. Other symptoms include anorexia and general malaise. Frequently a dog will show lameness that last from days to months first in one leg which will then switch to another leg.
You can read about Lyme disease here:
I hope I've given you excellent service. If so, I'd appreciate a 5 star rating. If not, or if you have any other questions at all, please reply here and I'll be happy to follow up with you.