Flat spot.
 

Hi i have a 97 uk turbo and when i boot it not all the time it will sort off hit a flat spot or die for say a second then carry on boosting but only does it in 2nd afm looks good and changed plugs and leads every year... does my head in sometimes. plus had a ecu check and nothing came back about the mass sensor..cheers karl

do you have a dump valve on if so try change it back to standard and try it.

As shaun said, stick a std dv back on. Or if you already have an OEM one, try borrowing a known good replacement (or buy off reBay)... Symptoms point to a leaking dv, thus ECU would effctively be overfuelling which can cause missfire, flat spot, etc...

Mine had the same problem only yesterday i put the standard valve back on and that sorted it drives great now nice n smooth and no popping now.:D