Bought from a garage and the problem started happening just after 3 months of owning the car (about a week after the warranty ran out - sods law). In 1st, 2nd and 3rd gear the car pulls just fine, every now and then I get a little stutter, like I have taken my foot off the accelerator and put it back on quickly. If I hit a duel carriageway/motorway and floor it after changing to 4th, it will go into limp mode around 65mph. If I change to 5th gear at 2500 revs (55-60mph, to prevent the limp mode in 4th) and floor it in 5th, as soon as it gets near 70mph it will go into limp mode. I currently get round this problem by bouncing the accelerator in 4th and 5th.
I'm was fairly sure it was sticky vanes so took it to a vag specialist garage who charged me £120 for clean the turbo out with chemicals (probably the £50 kit from eBay plus labour). Tested the car after I picked it up and the problem went away, a week later and the problem returned, £120 down the drain.
Now for the flaming.
I changed the air filter to a K&N. Pointless on a diesel I know.
I installed a chip from tdi-tuning. Cost me £120. I was happy to pay more than the cheap chinese ones from eBay as I believe they cause more harm than good. With the chip on or off the car has the same limp mode problem.
Unsure whether these points are relevant but it always helps to know the history.
Car has 105000 miles on the clock.
Where do I go from here?