Not Ranked
Possibly if the exhaust lobe on the cam is higher than the intake and this doesn't happen at lower RPM then you may need a stronger valve spring. I did see this happen on a car that some guys were building to drag race. It ran OK at idle and lower RPM but when they started to open it up the valves hit the pistons and completely wrecked their motor. They went to the cam manufacturer and they told them the valve springs they used were way to light and they needed much stronger ones for that cam.
And as Undy said, there could be some mismatched parts. Anyway I hope the fix turns out to be a simple and easy one.
Ron
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