Misfire at cruise, popping/afterfire at 3000RPM

@RustyRatRod It did, for the better part of 20 years from what I was told.

So, if I follow your logic, if an engine sits for a long period of time, some of springs would be compressed while others aren't, and some in between. And are you saying that springs compressed for long periods may experience problems? It makes sense to me, as in perhaps the compressed spring retains a memory and doesn't open up as much as the rest, leaving that valve susceptible to being left slightly open.

@AJ/FormS , give me a sec to read your post.

Yes. They can experience fatigue from being compressed for such a long time. Normally, it's not critical with hydraulic lifters because the plungers compress "some" but for 20 years, that would kill anything with the engine not being turned. I'd rule out everything else first, but the springs are certainly a possibility.