My R hangs revs for a moment then drops off when shifting, i have no codes.
It has absolutely nothing to do with the ABS module
5kg flywheel, running an M4.4 ecu & 608 bin file & its never given me any greif.
There is a super neat T-5R (M4.3) that im entitled to have regular access to that hangs for a considerable time, im just so used to my lightweight fw <3
There is a kit you can buy off a guy on Volvospeed that eliminates the "rpm hang" as its actually an integral part of the ecu that controls it & is not flashable, IF Thats what you are experiencing.
It makes sense that unplugging the IAC eliminates it, as the computer blips the valve a bit to keep the RPM up for that second, if you are fast enough or if you catch it at an odd time it will actually raise the RPM a bit too.
Do you have a dual mass flywheel in your R? Synonymous with an internal slave cylinder on the front facing side of the gearbox.
It's possible that if it is a dual mass jobbie, that the RPM dosnt stabilise or hold as the ECU wants it to as the dual mass flywheels are much heavier than the singles they replaced, the ecu gives it that bit of extra juice, exhibiting an extended hang condition or even raising it too much.
Have a read if this, he sells them as a kit and they are pretty spiffy, living without an IAC is kinda shoddy
http://volvospeed.com/vs_forum/topic/169473-venderbroecks-tinkerings/