when parked or idle at red light in gear rpm's will go from 800 down to 100

What is your question?

Part or in neutral or in drive at a stoplight the RPMs on the motor will go from 800 or 1000 RPMs down to 100 and almost all out I have a new alternator and a new battery and I am using the highest grade of gasoline the belts all seem to be tight so I'm not sure what else I can check please advise thank you