Probably it is snapping due to too much elevator. Try a nice large loop with very little elevator. If this works OK, but "rolls out" when you try tighter loops with more elevator then that explains it. Every plane has a point at which too much elevator causes a roll. You may get other responses that say to check that both elevator halves move up and down the same amount (good advice). Also check that one wing is not heavier than the other (also good advice). But I suspect it is simply too much elevator. Is it fairly heavily loaded? What size plane?