Yeah, that's what it sounds like to me.
I had dual valve springs in my '68 Firebird and it acted just like that. The bigger outer springs were broken, every one of them. And the inner springs were the only things keeping the valves from falling into the cylinders.
So check that. I'm glad you checked your pushrod length and eliminated that ugly possibility. Look really close at your springs and see if they're broken.
If they're not broken, then maybe they're fatigued...beyond their service life. You might try a new set of springs.
This still isn't making sense to me though, because if they WERE broken, it wouldn't rev when cold either. And if they're worn, I would think it'd be the same story.
...So I have to think about this.
I wonder if you're just detonating. That'll happen when it warms up but not when its cold, IF you're right on the ragged edge. Do you know what compression ratio it is? If you used really small chamber heads, you might have something like 12:1 compression.
An easy test would be to go get a gallon of really good race gas when you're almost on empty, and dump that in. If it runs great, then maybe all it is, is too high of compression.
