Larry, Check the pre-rotator clutch pads and friction disc. I saw this happen on an RAF some years ago. It may be caused by the clutch intermittently catching and releasing then catching again. Check the gap between the clutch plates; it may be too wide. I bought some Cessna brake pads to have on reserve to replace the clutch pads. Tom T. Hall in MN
Wow, that'd get my attention! If Tom is correct, please do nonetheless afterward tear down your gimbal head and inspect it for any damage/loose parts/etc. Having my SC rotor head to compare against the OEM RAF head, I am very leery of the Kindersley design.