Miss fire, under a load, warmed up only

I'm puzzled. I've been chasing this one for a while. New plugs, swapped ignitor, coil, distributor...
it lacks power more than usual and has a backfire and stumbles. I think the backfire is in the manifold. Runs great when cold.
I just pulled the plenum, and valve covers and did find three exhaust valve way out of spec. All intake valves are in spec. .009 to .013 is the exhaust spec. One is at .007, one at .0035 and one at .0025. The last two seem way too close.
Has anyone ever run into anything like this? Also, I changed out the knock sensor. I know it plays a role in all of this, but I don't understand just how. I've been trying to trouble shoot this and not just throw parts at it so if anyone has any info or ideas let me know.
Thanks in advance!!