irian jaya blue-tongue skink mysterious mouth/foot problems

Question
I adopted my bts in February from someone who didn't give him very good care....he's had mouth and foot problems ever since.  We see a very good exotic vet, who first thought it was mouth rot, but now thinks it is a yeast infection in both feet/mouth.  But every time we go off medication, he gets worse about 1-2 weeks afterwards.  We recently finished off a 21 day oral suspension to treat for a yeast infection.  Although he was looking great while we were on it, he is now going down hill again.

I am the curator of this guy and all of the animals at a nature center (raptors, amps, insects, arachnids) and have  about 6 years of husbandry experience with all of them.  So, he has UVA/UVB bulbs, he gets soaked 1-2x week, he gets insects, fruit/greens, pinkies....We take good care of him, but can't seem to shake this thing.

Any suggestions of what else might be causing his mouth issues besides mouth rot or yeast infection?

Answer
Mouth rot and yeast infections would be my first and second guess. It sounds like he's been in optimum conditions, the only other thing I could possibly consider is that another culture should be taken to see if there is any other form of infection or infestation.  I would also consult or have your vet consult the University of Florida for other insights.  The U of Fla has been very cutting edge in Reptile Medicine.  Also Dr. Mader may have an idea or several.