Bent Push Rods

I would check everything, especially on the top end. Cam specs, spring pressures, bent valves, all of it.

When I bought my '71 Dart it was running pretty rough. I took a look at it and found similar stuff, a couple of bent push rods, a couple of bent valves, a couple of pistons with valve strikes. Everything in the bottom end was stock with 80k+ miles, timing chain was new. I figured they replaced the timing chain when it went south and missed the valves and pushrods. Replaced two bent valves a few pushrods, buttoned it all back up. Ran great til I shut it down and started it again, timing was way off. Checked all kinds of stuff, ended up pulling the engine because it was pretty worn anyway.

When I tore it down I found out some mouthbreather didn't put a woodruff key in the timing gear when they installed the new timing chain. So the timing gear was slipping on the cam. The bolt was enough to hold tension when it was running, but on shut down or start up it was moving.

Good luck!