Your best bet is probably asking the question on either passionford.com or the RS Turbo section on the RSOC forum (bbs.rsownersclub.co.uk) as you're more likely to get a quicker answer on one of those forums and they're Ford/RS specific.
Either the metering head paddle is sticking or there is an air leak, if it has a dump valve first thing to do is to take it off as they just don't work on the s2.
