Motor runs but doesn't idle

[w2407]

1998 VT V6 auto, just had new motor put in by a wrecker. when he dropped it off it ran fine and a drove it around the block and into the drive way where it sat while i was doing some stuff for rego. probably there for a week then would hardly manage to start, then it started ok after i put some BP Ultimate through it (thinking dirty injectors or something) it ran choppy for a bit and conked out at idle. started it back up and it started to run perfectly fine. it revved smooth and i gave it a dose to clear it's throat, and it seemed to be ok. since then all i've done is wash it and de-badge and de-pin stripe, interior bits/switches/globes and a few other bits and pieces.
Now for some reason the motor will start and run but as soon as it drops down to idle it just dies, doesn't even pretend to be interested... i rang the guy who installed it and he said it'll be something small eg vacuum line from the throttle body. check them both and they seem to be connected at both ends and no obvious splits or holes...
long story short, the dude is giving me the run around and keeps 'no-showing' when i arrange for him to pick it up with his tow truck.

can anyone please throw me a bone here, should i be worried, or is it likely to be a quick and easy fix.
cheers helpers

[burnz]

are there any errors??
Just Commodores - For All Your Holden Commodore Needs

[BT1-355]

Sounds like your IAC motor is stuffing up