Thanks EcceCaspa, it did it on mine on the rainforest as well.
Only on Discovery HD and only when in Dolby Digital + Pro Logic IIx audio. I assumed it was my AVR having trouble with one type of audio. Then, however, I watched a movie on Starz HD that was in Dolby Digital + Pro Logic IIx and there were no issues.
Seems like it has to be the signal on Discovery.