I had a very similar sounding problem with some live-caught true perculas that I bought early on in my first tank. As time progressed, the inflamation/blockage around the mouth got worse, the fish got very lethargic, appeared to have trouble breathing and eating, and then finally passed away. Unfortunately, this happend twice in a very similar manner, with fish from independent stores. I always suspected some sort of disease, but it was very unclear, and I never found any disease ID that really matched what I saw.
The tank-raised true perculas that I got were very healthy from day one, and still continue to be.
Steve