I'm not sure where to post this but decided to go electrical. Hoping someone might have experienced this same issue and can point me in the right direction. Any ideas, suggestions or help would be awesome, thanks in advance. 400 small block recently rebuilt, 449 HP, quick fuel carb, headers, Ready to run MSD ignition set up broken in on the dyno and tuned, it idols at 900 RPM's. It has run perfect for about 6 months with no problems and now this issue. At idol, for no rhyme or reason the car will begin to "spark knock" or loose idol. It will drop from 900 RPM's to about 400 and struggle to run. If I give it gas it will come out of the issue and go back up to normal idol. This problem is not constant as it will only happen intermittently. It seems as though it's loosing power but none of the other components experience electric issues at the time. Things I've done. Tuned carb, checked power valve, adjusted curve on distributor, checked all wires for good connection, replaced spark plugs. I've checked the volts at the coil and when the problem is occurring I'm not getting a drop in voltage at the coil? I believe I have ruled out it being a carb issue?