Intermittant boost issue (i think?)

Guys,

I have a weird snag on my 97 A4 1.9tdi, (B5 AFN 110hp).

I will be driving along and the car feels as though the turbo just stops boosting?
Its not going into limp mode, no dash lights and no fault codes.
I'll take the induction pipe off have the missus rev the car put my hand over the pipe and you can feel the turbo boosting as normal.
Put the pipe back on and the car is fine???? for anywhere from 5-300miles then it happens again?

Any ideas would be greatly appreciated as I am at a loss.

Just got the car and have no history on it so don't know whats been done or not been done and have the manual on the way, so feel free to ridicule if its something simple and i'm just been a pleb

It's going to be pretty hard to diagnose without VCDS. Do you know anyone with access to it?

You can check the turbo actuator. When the car first starts it should operate the vanes. You can connect a vacuum pump to it to see if it will operate and if it holds say 10-15"/Hg. If it doesn't move could be vanes are froze or it's not getting vacuum from the N75. Lots of other stuff possible, that's just one interweb guess.

"Its not going into limp mode, no dash lights and no fault codes."

Just to be clear, it's not going into limp because no lights or codes?

How do you know it's not going into limp?

Limp mode and low power are so simillar it's hardly worth the distinction. I guess with limp you have full power sometimes, but the troubleshooting is pretty much the same.

Well, if you have power and loose it on a hill that points to the ECU reacting to some abnormal condition.

Low power alla time is something else.