idle issues with new A500 behind a fresh 318

As mentioned, it could be timing... Or a vacuum leak/lean at idle... Or it's possible the T/C lockup solenoid is leaking internally allowing the convertor clutch to drag..... If your thinking it's related to the trans get the power & ground hooked up, get an accurate tach on it & take it for a drive.. Lock the convertor & watch the tach looking for an rpm change...