Do you have upgraded valve springs?
I ran into a case a few years back overdriving my Banks turbo. Floored it up a hill, got 22 PSI out of it... and motor started ticking/loping. Turns out I bent a pushrod on the exhaust side, due to massive exhaust backpressure - it held the exhaust valve open enough to (evidently) let the lifter pump up, and then the piston smacked it and bent the pushrod.
Might the same thing be happening here - high pressures and weak valve springs allowing the lifter to pump up enough for the valve to just kiss the piston at that RPM. Under higher RPM, the lifters will pump down normally, so the added pressure may not be enough to hold it open enough to kiss the piston.
Edit:
Oh, and back then, I tried it again after upgrading the valve springs and... nothing.