As Niall said, could be any of a number of things. If MAF was a second hand one it could be that playing up. I'd try and get a lend of one and swap it for a quick spin and see if that makes any difference
Check fault codes, and if no code is stored, or anything related to idle control valve, remove the intercooler, and clean the idle control valve, and the throttle body. While the cooler is off, wash it out with petrol. That would be the first thing I would do.