Hm.. With the leg spasms and flipping my first guess would be something off with the neurological system. With them having only had him 5 months and probably purchased as a baby, they may be looking at something congenital that is showing up with maturity (sometimes congenital issues will do that; the animal will be fine, then get sick when it hits maturity). I didn't read every word of this thread, but like has already been said, I don't see anything in the keeping that should cause this.
What gets me is that the frog is still eating apparently; if you were looking at an infection or organ failure, I'd expect them to stop eating...