Sounds like you've done all the usual

Early VR6s with idle valve always seem to idle shit, mine in my mk2 is the later engine with stepper motor, no probs with the idle.

Only thing I can think of is a bit of a 'bodge' but.. If car does not have AC , there's a spare wire on the ECU that (when AC fitted) goes to ground with AC on , and raises the idle speed, if you ground this spare wire on the ECU it raises the idle speed. Seem to remember it being a blue wire, will see if I can find out which pin.