Does anybody know what would cause excessive wing rock during an elevator ?

Could be 20 different things.
You should take the time and post as many details of the plane in question

Poor design, warped wings, cg off (forward or aft), lateral balance, flying on edge of stall, etc. Take your pick.

I will start with moving cg aft.

Sometimes it's just the design of the plane. Some planes will *always* wing-rock in an elevator to the extent that some will fall right out.

Using a little throttle and a little more/less elevator can help sometimes not enough to turn it into a harrier or to pop it into a hover but enough to change the angle slightly so the wing is operating at a different AOA.

I would imagine everyone experiences wing rock to some degree. Every plane I've owned UCD, Funtana, Sukoi and now my EF 88" Yak had or has some wing rock. I was just working with my Yak today on harriers and it is my opinion the wind has a lot to do with it. Heading into the wind when the wind was blowing maybe around 10 MPH or less wing rock was minimal. When the wind died down wing rock increased. I added some power it stopped or was minimized. More power put me in a higher alpha but it did help. I agree with the previous statements CG has a bearing, and if your CG is where the manufacture suggested you may add some weight but a little at a time. A final comment or thought, wing loading probably has a bearing on it. If your plane is heavy, once wing rock starts it may continue to get worse. Remember not to correct with your ailerons you will only chase the problem. Use the rudder only to steer.
K.O.

CG and lateral balance will help most.I have also found that some props will react better and give extra thrust during low rpm,helping to keep the nose up.Sometimes not enough throw in the elevator can be an issue also.
Change it and fly it,repeat as necessary

To really diagnose the problem we need to know the airframe and power plant combination, total dry weight would help as well.

Your cg should be set to where the nose drops slightly during an inverted 45* upline. (Point nose 45* up, add power to not lose airspeed, Roll inverted, let go of elevator; If the nose drops to the horizon - you are nose heavy; If the plane climbs - you are tail heavy.) Move stuff accordingly and repeat.

You should have to hold slight pressure on the elevator to maintain level inverted flight.

77" goldberg Yak with a Brillelli 46 gt turning a 21x8 xoar prop. The plane is about 15.5lbs with full load of fuel..

15.5 pounds on 1160 sq" of wing is a pretty high wingloading for 3d and is gonna make stuff like harriers more difficult. A good 3d profile with that much wing would be in the 7 pound range.

The Goldberg 77" birds have too much wingloading in my opinion. I've flown their Edge, Extra, Yak. They all need 50cc engines to pull out of hover and end up weighing around 15.5-16 lbs, too much for that size ws. Moving the CG all the way back helps some, and the Yak is the most neutral airframe of the bunch. But they all wing rock at stall speed in the harriers, needing a tad of power to minimize it. My guess is that's why they all got discontinued, not that great for true 3d manuvers compared to the better 50cc airframes like EF and QQ with 10inches more ws for same class. Now, the new Reactor is 85" and pretty well priced. Wonder how that one flies in 3d...