94 pickup stumbling/hesitating

My 94 pickup (4 cyl 2WD - KA24E engine) is having hesitation/stumbling problems only after it warms up -- about 10 minutes into the drive. It does it at idle, acceleration, everywhere. I changed the spark plugs a couple months ago. After the hesitation problems I've changed the distributor cap and rotor, spark plug wires, fuel filter, air filter, intake air temperature sensor and the oxygen sensor.

I am getting a check engine light but the Nissan dealership wasn't able to pin point the problem because the light only comes on for a second and then shuts off. They gave me a list of parts that it could be and I've changed them all except for the air flow meter (mass air flow sensor) because that part is more expensive and I don't know if that will solve the problem either.

Does anybody have any other ideas before I spend $240 on the air flow meter?

I might have one because it might be something else with my truck and the one I had was fine. I hope to know in a day or so. Its off a 96 2.4 2x4.