I'm not totally clear what you are doing but you may be invoking the ECU into fuel cut on a very fine throttle setting.

i.e.

if your throttle switch

(IDL microswitch) is still closed

(badly adjusted?) with a whiff of throttle then you can cause the ECU to cut fuel if you are able to rev it too high with the IDL switch not yet open?

I've forgotten the revs at which it fuel cuts and I've forgotten the rules for fuel cut but it WILL cut the fuel if you are able to rev it with the IDL switch closed.

Once the revs drop to a lower level the ECU puts the fuel back on.

I'm pretty sure the fuel cut is somehere around 1600-2000rpm on a warm engine and it restores at lower revs.

If this is your problem then the engine will hit a precise rev threshold and the engine will cut fuel and the revs will collapse.

It will be fine and stable until you cross the rev threshold stored in the ECU

(IIRC there's a fuel cut map for overrun rpm trip vs coolant temp)

Then once the engine falls to a lower rev range the fuel comes back on and the revs will rise again.

If you don't move your foot then the whole cycle will keep repeating with revs rising to the trip point and falling again etc etc.

Maybe you should check your IDL switch adjustment.

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