How are you measuring the rpm? Had the same problem with a 69 stang. Seems after the third performance engine buildup the car did the same thing (it was a little more radical than the first two), only after I measured the rpm with a nonfactory tach did I see that the factory piece of s_it was reading 4500 and the engine was actually doing 7000 according to the SW race tach I installed later. That kind of explained the other two engines I blew out of that car (rods through the block), I was thinking the factory short blocks were just crap. Honestly I couldn't hear any difference inside the car. Not too proud of that one.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo