1994 Pontiac Firebird wont keep a constant idle

Hello my firebird recently had an entireengine replacement and for whatever reason now when I start the engine the throttle seems to go out of wack. The engine will start atabout 1500RPM's but then begins randomly jumping to 3000 back down and so fourth. After it takes about 25mins to warm up it will keep an approx idle in which the car is drivable. But being as temprental as my wife my car will ramdomly turn off on me. Which when driveing threw highways tends tobe very dangerous.I have also recently replaced the alternator radiator and belts since the car was parked for about 5 yrs since I was deployed to germany and then iraq. PLease help we really need a second vehicle and I already spent alot f money trying to get my hoopty running again. Sincerly a medic in need of a good repair guy!