My 496 used to break up REAL bad right at about 5500-6000.Electrical checked out okay,fuel pressure was not dropping and plugs looked fine(didn't appear to be lean)Talked to an old-timer down the street,first thing he asked was what size power valve was in my Dominator.I told him plugs,front and rear.He said put one in the front.Idle vaccum measured about 5.5-6" and I stuck a 4 in it.No problems since.I don't fully understand it ,but food for thought anyways.I have found in the past that ignition problems and fuel delivery problems act almost identical.Look at everything and don't get tunnel vision on the electrical system.JMO :D

I had that happen on a 358 one time. I switched distributors, wires, Msd,valve springs. Come to find out the spark plugs were too hot, range wise. Sometimes it is the simplest things. Good luck!!