RPM idle needle goes down and bounces back up?

I’ve recently come across this problem with the car at a standstill when the car is not in park the motor shakes and the idle needle goes down and bounces back up. It even shut off on me at a red light. I have a feeling it’s not the battery because the radio and the lights were still on. I eventually had to start the car again. It drives fine but when it idles it shakes. Please help