RevMax intermittant

Can anyone make suggestions as to why my RevMax is behaving oddly, please?

I've got a 100-100 mechanical setup, and the RevMax has been recalibrated on Aux2 of my AR7000/DX7

I have +60 and -66, set as my rpm endpoints for Aux2

In Normal mode, linear curve, the limiter works fine at 60

If I either:
a) flip to -66 (in normal) or;
b) switch to idle 1 or 2, (100% flat)

then, the engine revs to full throttle, then starts to surge and back off then surge and back off, repeatedly.

I'm confused and would appreciate any suggestions.
Thanks
Richard