Sometimes when I pull up to a stop or even park the car it begins to idle at a lower rpm and you can see the lights dim and it goes back to normal rpm then does it again. its continuous... normal idle then rough. (just pulling numbers out of my ass...) itll go from like 500 to 300 for a sec or two then back to 500 for a sec then down too 300. It isnt anything wrong with the motor because I replaced the motor 1,000 miles ago and it happened with my old motor. Thinking it is maybe the alternator>

 

If it happened with ur old motor. Ask yourself, what parts do you still have that are original. Then focus on those.

You sure this event does not occur when your cooling fan comes on to regulate the car temp. That bogs the idle down for a split second and then subsides due to a properly functioning idle air control valve.

 

The only thing that swapped was the motor. not the alternator or fuel pump or anything. And it isnt that. It is not normal. Ill try and get a video if it happens again.