Does it misfire at all? Just thinking out loud, my iacv (stepper motor) has stuck open on occassion, causing misfire and sudden drop in revs/loss of power when coming off the throttle. I managed to give mine a good soak overnight and refit successfully restoring it to normal operation. Appreciate you've fitted a new one, but could be gunk in the iacv idle adjust port interfering with flow. Have you tried any maf/carb/egr cleaner in there, and the maf etc? The other thing to check might be the vacuum hose on the fuel pressure regulator, although a leak here would presumably cause more constant issues.
