snap-rolling out of a loop is usually a tip-stall situation. 2 basic causes:
1)You apply excessive elevator and stall the wing, even at high speeds. (probably not this case)
2)You lose too much speed before going over the top.

If you've got enough power for the weight of the plane... try a longer, lower pitch prop. This will reduce the loss of speed.

the problem can be agrivated by having the plane set up with cross correction of rudder and aileron. Go to www.nsrca.org for the correct way to ensure a plane is trimmed.