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Question: volume keeps increasing by itself (Onkyo HT-S990THX)

I have had this system for 2 years, I work in the audio industry so know alot about music and sound, and this is a great sounding system. I have a problem that has just appeared - with digital audio (toslink) coming from my satellite receiver, sometimes when switching from commercials back to program, the audio slowly starts increasing. By itself. I have to keep an eye on it and decrease. Anyone heard of this? Thanks, John.