Yamaha F9.9BM with strange problem

Hi, I have an F9.9BM (approx. 1990-model, I think) that does the following: Starts fine, runs for several seconds up to minutes, until it takes on a very irregular "cyclic" high idle. Seems like the ignition is turned off and on in cycles of 1-2 per sencond. It does not feel hot when this starts, cooling water barely lukewarm. If stopped, it sometimes will run OK for a minute or two, before the symptom recurs, or it can be started and the problem is there right away.
- Plugs changed, correct (CR6HS) with correct gap (0,65 mm).
- Valve clearances correct (inlet 0,2 mm, exhaust 0,25).
- Fuel line OK, vent in tank open, hand pump OK, fuel pump in the engine OK, problem not reduced by hand pumping the bulb-pump.
- Engine oil level and appearance OK.
- Ignition spark fresh, sparks over 2-3 cm gap when disconnected from plug).

To me it could seem to be some kind of electronic protection system interfering? Any suggestions?

[boats.net]

May have an ignition component breaking down.

Problem solved: Low oil pressure. Problem "disappeared" when electrically disconnecting the oil pressure switch. According to the manual the engine has a safety function that cuts in when the oil pressure is low and limits revs to a high idle. I have no oil pressure gauge available, but I have cleaned and pressure-tested the switch, and it seems to be OK. So I guess the root problem is low oil pressure, sometimes getting low enough to engage the safety system.