508 or 509 Purple Shaft cam

I have a air gap with a 750 holley. This is the second motor for my car. but the first motor did what you are saying. it had a 508 purple. i disconnected the vaccum advance it halp some with the rpm problem. It did it coming to a stop mostly. had to smack the gas petal in neutral to get the dam thing to come down. You could pull a valve cover and measure with a 1 inch indicator on the bottom of the rocker and it might get you close.