Why would the RPM be high on a car when your not going fast?

Well if you drive a manual transmission vehicle, it could be that you haven't up-shifted yet, or you downshifted before the RPMs reached the upper limit for the lower gear, causing it to over-rev the engine. If your vehicle is an automatic transmission, your gas pedal could be getting stuck, or your Idle Ar control valve could be stuck in the open position.

I of course would need more information about your problem. Please re-post when this happens, or and special conditions it happens under.