I am having a similar problem with a couple snakes. It was first noticed in an older snake which I brought home from a show but I beleive I have narrowed the source down to some type of parasitic infection from food. I had to get some bigger mice/rats from another source since I didn't have any big enough to feed this snake in stock. The mice were not professionally raised and were usually prekilled as opposed to frozen and thawed.
I did a week of oral treatment and injections every other day for 10 days but symptoms are still the same. I forget exactly what it was, Flagyl or something, although I will doublecheck and update you. Whatever it was he said it would not kill parasites so I think this will be the next step. Unfortunately I am still waiting for my vet to return from holidays this month before I can bring the snake back again.
Your symptoms sound similar - Sometimes bubbles could be seen but never very much mucus or anything like mouthrot. Raising temperature didn't do anything at all and I was worried about humidity also but was told from different sources that this shouldn't be too much of a problem. I am currently at around 60-65 humidity (rainy weather lately) and have added a dehumidifier to my room just to be on the safe side. Tanks and water dishes have been exceptionally clean lately also.
For mine I'm suspecting some type of lungworm/hookworm or something althoug I have zero experience with reptile parasites. Any other guesses from anyone?