My Volvo MD2030 diesel engine experiences a problem since last month.

Normally running at about 2400rpm (that's my cruise rpm), it suddendly
goes down of rpm, approximately at the minimum rotation speed, for few
seconds. Then, with no apparent explication, it returns to previous rpm
rotation.
This happens periodically. Turning at lower rpm (around 2000rpm), it
happens once per hour. Turning at higher rpm (around 2600rpm), it
happens every 10'.

Even if I lower the throttle, it doesn't shut down. It simply reduces
rotation exactly if I had lowered the throttle, but with no
irregularity nor noises nor smoke.
That's why I don't think it is some kind of overloading.

Water flows perfectly at a very normal temperature (I can keep my hand
in the hot water huge flow).
Smoke: just a very light white smoke, disappearing after few minutes of
operation.
Leds in the cockpit are off (they light on at the led test).

I changed oil, oil filter, and mainly Fuel filter, with no result.

My opinion, but I'm not able to find any confirmation, is that some
kind of 'overheating thermostat' is going crazy, and it reads a wrong
temperature, lowering down the engine to protect it. Then, after few
seconds of low rpm, it cools and the engine becomes ok again.

Any idea/suggestion/confirmation?

Alberto

--
"Se viriamo, viriamo viriamo viriamo;
Se andiamo dritti, viriamo"
(Cit. "il tattico")