I've got a 2000 accord 4cy auto and when the the ac/defroster is on the and the car is at a stop it surges what could be the cause of this

 

Alternator, ground. ECM/PCM, battery......the load of the defrost is causing the ECM/PCM to adjust. Now the question is what/which......several tests to get there.

Me, I'd start simple. Check the gounds to be sure they are clean and tight....and have the battery tested.

From here, you'll need a voltmeter, a good manual and some time.