ADCOM GFA-5800 clipping

I just purchased from a friend an ADCOM GFA-5800. Can anyone shed light on why this Adcom GFA-5800 clips even at a moderate volume level? Could there be a problem with its thermal protect sensor? For such an above average rating of 250 watts per channel, why does it clip despite being driven lightly? The problem presents itself consistently after thirty minutes of use at a decent volume level. The amp doesn't even get hot yet it persistently clips(left channel yellow light glows). I've checked all connections, speaker wiring and another power amp hooked up with the same system as the 5800. The other power amp doesn't clip. Any info on this would surely be of great help. Thanks.