2001 Maxima High RPM After

[2001I30MAX]

My 2001 Maxima with 177k miles has high rpm after, say, 10 minutes of driving. When the car is started, it has normal rpm around 700 and after driving some miles, it jumps to close to 1000 while it is in 'D' while waiting for traffic light and to around 1600 after I park the car and put the gear to 'P'.

When I let it idle for about 10 to 15 minutes on my driveway, the rpm doesn't hike up, and stays around 700. This only happens when I am driving.

About a month ago, it happened occasionally but starting a week ago, it seems happening all the time.

Does anyone know why this might be happening? Would this be a warning of a part malfunction that's forthcoming? I don't have any SES light/code so I don't know what to look or what to do.

Thanks for your input in advance.