Good point. Even if it were in the wiring it would do it in all gears especially. 4th gear tends to point in the direction where its when the engines at its greatest load. I might even test the alternator at this point and if an oscilloscope was available check the spark curve per cylinder.. Something is screwy somewhere. I remember having an 86 bronco with the EFI 351 and it did something similar and turned out to be the TPS.. man that was a biotch to find until we hooked the TPS to a scope and saw the glitch right away..