Mine did that when the timing was screwed up...one tooth off on one of the cam gears, so we pulld it and tried again, got it...how long does the lag last? where did the ECU come from...Damn computerized stuff anyhow, you may have a glitch somewhere that is telling the injectors to fire just a little too far behind the opening of the throttle... That's just off the top of my head...I can look in my books a little.