My symptoms are:

Astra year 2000 with Engine code Z18XE runs perfectly, but revs fall every 2 seconds and then return, for example, if the throttle is set to 4000rpms then engine will keep running but revs will drop to 2000 for 2 seconds and then return to 4000 again for 2 seconds and then back to 2000 again.

All this happens without the butterfly valve moving (or the throttle pedal being pressed). My temperature gauge reads zero, even if i drove the car for a week non stop. Whether this is correct or not working I'm not sure.

I'm getting codes 0170,0100,1555,0110,0130 (o2 sensor), 1525(limp home), 1550(electronic throttle control reduced power), 0115(engine coolant temperature curcuit high temperature).

Throttle body has been removed and cleaned thoroughly, (including very small hole).I did notice an improvement in throttle response after this The 0130,1525,1550 and 0115 only showed up after the throttle body was cleaned?

I've also changed the spark plugs, the old ones all looked ok and in the same condition as one other.

I’ve been told the problem could be the ECU as the engine symptoms could not be caused by a broken thermostat etc. Thoughts?

Unbelievably a new thermostat is £200!, but that is still cheaper than an ECU

Help would be much appreciated