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Power Problem

This is a '92 Grand Prix SE 3.1 MFI, it has recently been having troubles with the power from the engine. Namely, it feels like I have only three cylinders part of the time. It gives me full power sometimes, especially at high speeds, though the vehicle will shudder between 2200-2500 rpm.

I have replaced the Cat, Plugs and Wires, and checked on the starter coils and vacuum tubes.

Nothing I do seems to help, any suggestions would be great, short of junking the car.