It's also possible that you're stalling the wing a little. While I do agree that the servo or linkage would be the first thing to look at, that kind of plane can have a fairly gentle stall, so it's possible that you're losing a bit of lift, which opens the loop, picking up a bit of airspeed, which allows you to tigten the loop, and repeat. I've actually seen that exact kind of thing with some planes.

Try playing with the throttle through the loop. I don't know what you're doing now, but try looping at higher and lower throttle settings, and using less throttle on the downline side and so on. It won't "fix" anything, but it might help you get a feel for exactly what is going on.

Also, try moving the CG back a little. If you are nose heavy, you need more elevator to rotate the nose up, and your stall will tend to be more gentle and can help cause the kind of thing you're talking about. Moving the CG back might allow you to snaproll out of the loop though, so be careful, do it a little at a time, and test-fly and test the elevator response at altitude after each change.