Not sure it matters, but in the beginner and next step up bank preset defaults there is some collective expo dialed into the preset defaults. On 3D and up it is set at 0 expo.
Other things besides cyclic gain it could be:
0) As Tony mentione, check for binding in the elevator linkage or other linkage and swash. Could be a servo binding too.
1) Blades torqued down too tight or too loose
2) Blades on the wrong way (tailing edge leading): the pitch would be right, but they would tend to grab the air above and below zero pitch and cause some odd flight behavior.
3) One blade is seriously out of zero causing a large blade tracking problem
4) Blades unbalanced
5) Blades damaged
6) Rotor or main gear jesus bolt loose. Or excessive axial play in the main shaft.
7) Swash damaged or swash bearing loose.
This is all assuming your etalking a collective up/down wobble as opposed to an elevator wobble, which would be cyclic gain or perhaps 1, 3, 4, or 5.
