I've done a bit of hunting around and others have reported similar problems to mine, but nothing seems to match exactly. Iv'e been having problems for a while, so I'll try to be as specific as possible with what I've done so far.
Firstly the car started having terrible trouble with power loss. It would run lumpy at idle and properly struggle to get up the 1 in 4 hill I live on. Stopping, switching the ignition off and on again would get me up the hill, but still low on power. I had the MAF changed, didn't sort it. A throttle body clean eventually fixed the power loss, but not the lumpy idle. In an effort to get the idle fixed, my mechanic changed my divert valve with his, used but known working, part. Doesn't seem to have fixed the issue.
Since the divert valve was fitted, I've noticed a dump valve sound when lifting off the accelerator. Doesn't bother me all that much (it's even quite fun on occasion as chavs in their Saxos go visibly green when it happens right next to them), but I'm wondering if it's related to the main problem I have.
The big one is that the turbo seems to be sporadic. It is most noticeable when the engine's on full chat, particularly in second and third gears. It comes off and on as if I'm pumping the throttle. I can't hear any louder hissing noises than the usual turbo sounds and I'm not sure what I should be looking at. I know some people have experienced problems with the sensor on their airbox (N75?), but mine's a facelift car and doesn't appear to have one. I've also heard that the breather pipes are prone to crack and split, but people with this problem seem to report a more constant loss of power than my off-and-on issue.
It's a 2001 1.8T and I hope somebody can help point me in the right direction. I'm not a car novice, but this is the first turbo I've had and I'm at square one with it.