Help! Sputtering/hesitation in gas

98 Jeep Cherokee Classic, 88000 miles, 4 L.

We've been having this problem for a while, and our mechanic has not been helpful. After starting the car, as you start to drive it, especially out of a stop sign, the car hesitates and sputters. It feels like as you press on the gas, it doesnt give a constant stream of gas into engine, and the car kinda jerks around. This tends to only happen when the engine has not warmed up all the way, so we've been letting it idle for a minute or two before driving, but it still sputters. Also, after we bring it home from the mechanic the problem comes back and deteriorates over time.

The mechanic we've gone to has no idea. We've had the distributor cap replaced and the O2 sensor replaced. We've had the computer checked and the fuel pump checked, and those seem in working order.

Help!