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?
Are you trying to run 2 speakers while your're bridging it?
No. I have several 250 W, non-inductive resistors I use as loads. When in stereo I connect one 8 Ohm resistor to the Left Channel and another 8 Ohm resistor to the Right Channel. When in Mono-Bridge I use one 8 Ohm resistor. When in Mono-Parallel I use one 4 Ohm Resistor.