Just returning to my post to put in a follow up. I did indeed have to get the main board replaced. So far the problem has not recurred.
RickySamosa, I would agree with fflbrgst's advice. Try a factory reset, and make sure you're running the latest firmware on both amp and shortboard (if you have one). If your problem continues, get thee to a service center, as it's not likely to resolve itself. The master volume pot is the only "analog" control on the amp (I think), and it may be something as simple a flaky pot or solder joint. Good luck.
stevedenney's post in Spider Iv 75 - Volume "sputtering" Fluctuations was marked as the answer