I was going to recommend your CIS thread msh.
http://www.classic-audi.co.uk/forum/...ad.php?t=29491
Or isham research, or the sjm website, because it could be bloody anything!
My 'guess' was a problem with the differential pressure regulator on front the metering head, or it's control. As it and all other mixture/emmisions related stuff are being bypassed, (and mix changing from 14 parts air to one fuel, to twelve air to one part fuel - ish) when the WOT switch is depressed. But just a guess!