I have seen this behaviour in DEC before. It happens more often that not when your scope is on one side of the meridian and pointing right at the zenith. It bounces N/S because it is "hanging down".
Couple of things you can check.
1_ Do you have lateral axial play in your DEC worm.
2_ Is your gearbox shaft to worm coupler secured without rotational play or flex.
3_ This is extreme but older G11 had a bronze bush only (no bearing) mating with the DEC spur gear. So if there is lateral play between that gear and the bush then that can contribute to lateral axial play. It's easy to check. Mesh the DEC worm to the max then tighten the clutch to the max. Move and rotate the DEC saddle plate by hand. If you feel any "click" it's likely something's giving other than the worm.
Anyway check that first and report back see how you go.
