1996 Chevy Truck fast idle

I had my alternator go out and once I put in a new one now when you start the truck it revs to about 2000 rpm and then slowly goes back down to 700 rpms, and sometimes when at a stop sign the motor will rev up to about 1000rpms by itself.
I dont think it has anything to do with the alternator.
It has the vortech 5.7