Last summer when the temperature in my tank soared (over 86F, that's as high as the therm. goes) I lost all my java fern. It all turned black and started melting. When the temperatures cooled, it all grew back.
I also noticed some turning brown and sickly a while ago, it looked very similar to when the temperature was rising but the temp. was only 76. Increasing circulation helped a bit, moving the Egeria densa out of the tank seems to have helped as well, although it could just be coincidence.