Hey mate,
I had similar problems with my 88 GQ petrol/gas 4.2 carby. I found the temp sensor was loose and giving false readings, I soldered up the sensor and now it works fine, just reads slightly higher than standard because of the extra resistance through the connection. I still had a cooling problem though with the coolant overflowing regularly on my trip to work (which is 50km's + btw). I had the radiator out and flushed it out and found it was 10%-20% blocked, I then had the whole system pressure flushed and a new thermostat housing and thermostat fitted. The system now runs fine and cycles at the correct temperatures. The only time I have problems now is extended periods at 100km/h plus, when the guage will creep up. If anyone has any reasonsfor this I would like to hear them but at the moment I assume its just the 35's and a few too many spotties at the front that probably dnt help either.