sometimes you won't notice a valve problem untill you rev it up a little bit. As a fact thats how I got my engine. the guy was having problems with it backfiring through the TB, tried a lot of things and couldn't fix it, ended up pulling it for a crate motor, and selling the old one to me for $200. The problem? an Intake rocker arm pushed though it self to the valve stem. Moral of the story is I had got an engine with 60,000 miles, needing a $2.50 fix.

HOWEVER, you have a timing problem, I had a similar problem a while back, I changed the distrib. and reset timing, no problem sense then. timing is a 4 degrees advanced on the balancer. I would tell you total timing, but i never learned how you figure that out. sorry