hey i have a 1991 dodge stealth dohc v6 and when i get to a light the rpms go down to about 1000 like they should and then it sometimes drops to about 500 and when i shift from 1 to 2nd there was one time it missfired. i just got an engine from japan and the engine had been setting on a shelf in a warehouse for like 2 years so is it just bc its new to my car or what??? please help thanks

The IAC may need cleaned or replaced. The ECU might have leaky capacitors. Plug wires could be bad, plugs as well.