Intermittant power loss on 2006 Polo GTI 1.8T

[magouster]

Hi all,

I've just bought a secondhand 2006 Polo GTI 1.8T with 55000km (34000 mi) on the clock. When getting hard on the gas, the turbo spools up perfectly at about 2000RPM, but power dips at 3000, at 3800, at 4100 and then again at about 4500 RPM. (All this in second gear. In other gears the RPM spacing is different, but it happens every 1.5 seconds or so no matter which gear is selected.) Every time it loses power, it actually sounds like the dumpvalve releases some boost pressure, and then builds up the pressure again.

I obviously know this is not normal, but I'm wondering what the possible cause could be. I'm yet to take the car to a VW dealer for them to do a diagnostic, since they reckon it could be a electronics problem more than a mechanical problem.

Anyone ever experience something like this or know what could be causing this?