My 1978 280z/Auto usually runs awesome. Very strong, then all of a sudden, usually when its warm it skips. Out of the blue it starts skipping, then I'll be driving along at 2500 rmps and the skipping stops, I can feel it clear up and its back to running strong again???? I replace all the injector seals and cleaned the terminals, new wires, cap, rotor, plugs, oil, adjusted the valves, new gasket, new fuel filter, fuel pressure is good, I hear a hissing noice coming from the EGR valve though....any thoughts guys?

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1978 DATSUN 280Z
1974 Lincoln Mark 16k orig
1949 Studebaker
1970 Buick GS 455 frameoff(sold)
inventory of Z parts 75-76