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Its probably a driver problem as tri-sli crossfire is never supported as well as dual cards. What psu were you running these cards on?
Your brother is gonna be a happy man.
I am running Tri GTX280's and I have never had a problem like that, could be that one of the cards wasn't happy in TRI SLI?
Adaptive power management can do that... however if your testing just Black Ops then this isn't an SLI problem but one of the many issues with black ops, the game has terrible rendering issues with a variety of platforms, I can't run more than 3 maps in a row without restarting the game or I get extremely varied framerates on the 4th map.