You may need a new water pump, or some other belt-driven accessory at that mileage, but I don't think the water pump is causing your driveability problems.

It could be a variety of things, but the check engine light plus the stumbling after the engine has warmed up suggests that it might be an O2 sensor issue. (Among other things) Try testing your O2 sensor and see if it gives you good, snappy voltage changes. I can't remember the exact values but I think it should alternate between 0.2 and 0.8 volts once or twice a second, with a sharp voltage switch between the two.

Anthony