1999 Cadillac STS RPM revving unusually high

Hi there, the past little while i'venoticed my check engine light come on and as I was pulling into my driveway I released the gas pedal but the car was revving at around1,500 rpm instead of the normal 500 or so, therefore with no foot on the gas, my gar wad driving alot faster than normal. As I put the car to reverse it had a very hard shift and still was at 1,500 rpm. I put the car in park and shut off the engine, waited about 10 seconds and started it again and it was fine. This has happened a few times the past couple of months? Could it be I need new plugs and wires? I did take the car in to a machanic and he told me no codes were found and reset the check engine light. But he did say the engine has a slight mis-fire and would be a good idea to change the plugs and wires eventually. I just dont have any idea if this is related? If you have any suggestions thanx RG