stumble problems on 1990 Nissan 300ZX

Need Help any ideas welcome and Thanks ahead of time! Ok I have a 1987 300 zx non turbo, v-6, 5 speed. New oil pump, timing belt, plugs and wires cap button, new head gaskets worked heads intake upper and lower etc. Ok here is the problem it stumbles between 1400 and 2500 near to the point of shutting off. Acts like (A) lack of fuel or and (B) lack of air. Cleaned mass air flow sensor, Checked Belt timing, Checked distributor timing, set at 20 before top dead center, Checked idle set at 700 rpm, Replaced all Vacuum lines, Checked to make sure no vacuum leaks, has 35-45 p.s.i. fuel pressure. Has good exhaust pressure, will idle and rev without stumble but under load as driving stumbles.

by in Warner Robins, GA on December 10, 2012
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2 answers 3 comments
ANSWER by , December 10, 2012
Did you check fuel pressure under load? or idling?
COMMENT by , December 10, 2012
Had the upper and lower Intake put in the Vat when had the heads reworked. The warmer the car gets the more it stumbles. Once above 30 mph all seems ok. Had the injectors cleaned also. I just rechecked the Belt timing all is good with it. Replaced the FPR also. Checked the compression all with in few psi of the same. Checked the TPS in specs. Now I have not checked the fuel pressure as driving but seems good as I rev the engine parked. Thank you!
ANSWER by , December 11, 2012
Possible EGR malfunction. Disconnect it and test drive, if the stumble goes away , diagnose the CAUSE of it's improper function , don't just replace the valve , it may not fix the problem.
COMMENT by , December 11, 2012
Agree, EGR is possible.
COMMENT by , December 11, 2012
Ok the codes I am getting are the idle switch and crank sensor and 0/2 sensor and detonation sensor Thank you!