If you're running split elevators, make sure the travel on both halves is equal. I had the same problem on an Extra 300. Mild loops were okay, but hard loops with 3/4 to full elevator produced a snap roll due to a little more travel on one elevator half caused by the side by side dual NyRod setup at the servo. For equal throws, the tubes should have been over/under to eliminate differential travel differences.

Dual, unmatched elevator servos can cause this as well.

I'm talking SMALL differences in throws causing the problem. You really must look at it carefully.