The easy way to rule out the G-force question is to try the same maneuver, only inverted. Rather than apply up elevator, from the inverted postion apply down elevator. This will cause the G-forces to act exactly opposite.

You never mention throttle setting when you are performing the maneuver. Are you at full throttle? Does the engine exhibit the same behavior from a lower throttle setting (i.e. half throttle). Is there a threshhold throttle setting above which this behavior occurs?

I ask because it could be related to throttle linkage. If you are cutting the throttle at the entrance to the maneuver, there could be flex in the engine mount or the connecting rod that is causing the throttle to be pulled further closed, causing the engine to starve.

Brad