MC2150 Mono Mode Problem

I'm having a strange problem with a MC2150. If I hook it up in Stereo mode, I can get 165 W out with both channels driven into 8 Ohm loads. If I switch to Mono-Bridge and connect an 8 Ohm load between the Left and Right 4 Ohm taps, I can only get 78 W. If I switch to Mono-Parallel, jumper the Left and Right 8 Ohm taps, and connect a 4 Ohm load between the common tap and the jumpered 8 Ohm taps, I can only get 15 W. I called McIntosh and spoke to a technician, however he could not recall anything like this ever occurring. Does anyone have any ideas?