This sounds like there must be an air leak somewhere or the throttle's not closing properly for the engine to be reving up without the IACV connected. I would try adding some slack to the throttle cable, just in case it's being pulled open for some reason. Otherwise, you're looking for air leaks round the inlet manifold.If i start the van then unplug the IACV it runs lovely to start with but then begins to rev constantly fast as the van warms up.
There are a couple of different designs of IACV that VW used, but anything labelled as from a Golf GTI of the same vintage should work. Part no is show as 1 here, and a new one can be had for £50 or cheaper s/h on ebay.I have looked at IACVs on eBay and found that VW ones run from around £25 - £150. Do I need to get a T4 specific valve or will any VW one do? they look similar but slightly different shapes so this leads my to believe they are not all interchangeable. New ones from VW are about £150 and I'm loathed to spend that much if it's not necessarily just that thats the issue.
Good luck in getting it sorted.