91 daytona idle problem



I’ve just got done putting a new motor (v6 3.0l) in and just had to fix the timing belt because i put it on wrong but now that good. when i start the car it starts up perfect but after a few seconds it revs up to about 2500 rpms then down to about 300 then back up and down with no set time it could be 5 seconds or 15 seconds but no more. then it dies. ive messed with the distributor cap trying to fine tune it and it just makes it worse. is the timing just a hair off, vaccuum, etc?