Maybe a slightly bent valve causing the valve to hang open or stick closed.
Thats my 2 cents.
You also mentioned you straightened the pushrods. I would never run a restraightened pushrod, you are asking for trouble.
Kev, I was thinking along those lines also. I pulled the heads and everything looks good. I removed the springs, and all seem to be in good shape along with the valves. None sticking for sure. Do they all seat properly? I am still looking into that. I have to reassemble and test with a liquid to see if any are leaking. I hear you on the pushrods and am going to order new ones before I reassemble the engine. Thanks!
