If the engine is not reving, just the gauge moving, then most likely gauge or intermittent open circuit.
You will need to take to trusted garage or dealer for further diagnosis.
They can monitor PCM RPM output to see if signal is bad or gauge is problem.
If engine is reving, then you may have a transmission slipping. Get that fixed ASAP, as it will only get worse.
Saturday, December 12th, 2009 AT 7:56 AM