It's probably your dampers too tight.
Where the feathering shaft goes thru the hub there are some o-rings and some tiny shims and some step washers. All this stuff has to be assembled correctly. The tiny shims can make a big difference. I have seen replacement hubs that needed only 1 shim per side, and up to 5 shims per side is not unusual. If the damping is too tight, that is too many shims, or if the step washers are not in correctly, it will "nod" or "twitch" like you are describing.
Also severe blade imbalance will do that but I doubt if that is your problem.