It's impossible to diagnose something like this over the internet - it's an internal problem after all. The cold has been known to cause problems for dart frogs and that's why I haven't been shipping since early November. I've read the thread you started about this on dendroboard and I think you got some sound observations, except Susan (Earthfrog there) likes to blame everything on chytrid. I sincerely doubt it's the problem. Get the frog to a vet or play it by ear.