MY 1992 VP Commodore V6 starts fine, but when it reaches operating temperature, the engine surges and revs rise and fall quickly in drive or neutral, though it doesn't stall. I have replaced air and fuel filters, spark plugs and leads, cleaned the throttle body and idle air control valve to no effect. What is the problem?
I WOULD look at the oxygen sensor in the exhaust system or a temperature sensor. From what you describe I think your engine is running rich. Either the computer isn't getting the message that the engine is up to temperature and needs a normal mixture, or the oxygen sensor isn't working.