I think it could be one of a couple of things.

It could be a mild stall:
Basically, you increase the AOA of the wing when you pull the elevator.
This causes an increase of lift as you'd expect.
But you also get a BIG increase in drag, causing airspeed to fall off.
This loss in airspeed leads to a mild stall, causing a small loss of lift which causes the nose to fall and the loop to open up.
But when the loop opens up, the drag goes down, the engine pulls the plane to a higher airspeed, so the lift goes back up, and the loop tightens.

It could also be the elevator getting in to the wake of the wing. Basically, as the nose goes up, the tail drops. There is a downwash of turbulant air coming off of the wing. If the elevator gets in to this air, it becomes less effective. So, the elevator looses effectiveness, the loop opens, the elevator regains it's effectiveness just long enough to get back in to the wings wake, and the cycle repeats. (I actually think this is less likely than the first though)

For a solution to either one, try a lower pitch prop. This will allow the engine to pull the plane at a more consistant airspeed though the loop rather than slowing quite as much as you pull on the elevator.

You can also try moving the CG back a little. This will make sure that if you ARE stalling, it will be a lot more obvious . Actually, what it will do is make the elevator have to work less to pull the nose up, making the effects less obvious.

Also, try easeing in the elevator. I don't know, but I'm going to guess that you start your loop pretty suddenly, going from level flight to full deflection in one movement. Also, as the plane gets vertical, ease off the elevator, and let gravity help you over the top of the loop. A nice round loop often requires adjusting the amount of elevator you hold though the loop.