hesitation

87 Z28 5.8 TPI hesitation problem @ 25mph. what could it be? Fuel injector/pump, head gasket, distributor, timing chain???. Recent tune-up, changed plugs, cap and rotor and wires again. Also, the fuel filter was changed. Car does not die out with the A/C turned on. Reset the timing (without the ECU plugged in of course) Seems like the timing changes once in a while so could that mean its the timing chain? Several people have told me that is rare to have to change it. One person said it was the head gasket, another told me the fuel injectors.

Thanks in advance