1996 Ford Escort Sport, 123K miles, 1.9L

I have a 1996 Ford Escort Sport with a 1.9L engine, 123K miles that recently started running a bit rough. I got a check engine light---checked oil, air filter, trans fluid, coolant--all is well. The temp guage is in the normal and nothing electrical seems to be out of the ordinary. When the engine is cool it seems fine but after running for a few minutes it seems to stumble while idling. Once I start driving, it seeems like at times it's not getting enough fuel. It will run fine, then stumble on accelleration, then surge back to normal for a a few seconds, and go back and forth between normal and stumbling. I replaced the fuel filter and noting improved. I replaced the alternator about 10 months ago, along with a new serpentine belt and tentioner. It's going to cost me $80 to have to diagnostic run, and I could replace the fuel pump myself if I knew that was the issueany ideas?