In reference to what David C says:

I had a similar problem. I'm the type of flyer who likes to throw them sticks all over the place (mainly from trying to get a trainer to do aerobatic/3D stuff). I moved from the trainer to a 3D plane. I had the same situation (loops snapping out). What I thought was a problem with the plane turned out to be my flying. I had to learn to take it easy on those sticks, ease into the loop, and no more slamming on them sticks.

If you think this is your problem, try this:
A very good flyer at my field (probably the best in the region) told me to practice flying helicopters on the simulator (that will soften my controls up, smooth them out), and it did work...Now I can make my loops as tight as I want without snapping out.

and/or:
If you have a simulator to play with, just pick a 3D plane (extra, cap, edge, sukhoi (my favorite))and practice tight loops until you get the hang of it.

Good Luck!!!