Boost issue, please help

First I would like to say that I've tried to search for this issue but I couldn't find a problem like mine, maybe is just me.
The car is 1999 Golf TDI, 5 speed manual.
I do have a boost pressure gauge and a boost valve, limiting the trubo's boost pressure to 18psi installed by the previous owner.

For the last few days the boost just cutting off. If I start accelerating the car the boost gauge showing increase up to around 5psi and after that just cut off and goes down to 0. One way of resolving the issue is to stop the car and restart the engine. I re-checked all the vacuum hoses and re-installed them now they are tight and with no leaks (I tried with soap water) however the problem still exist. In the morning when the engine is cold, no problem at all as soon as the engine get up to normal operating temp... here we go. On the long runs I don't have issues maybe because the engine is working above 2000RPM?? I don't know please help.
Thanks
Antony

Anyone??

Help please

I would ordinarily suggest a vacuum leak for power loss. How do you check for a vacuum leak with soapy water? It won't bubble like it would for a pressure leak, will it? But if your boost gauge is right, that means the turbo isn't pressurizing. How can that be? Maybe with a bump someone else can help.

I just thought - could the aftermarket boost valve be sticking open?

So you are saying when the engine is cold the boost gauge indicates all is well ie sustains the boost? Then when the engine warms up, you cannot hold boost?

If this is true I would suspect N75 as it is an electrical solenoid.

If this is not true I would try removing the boost limiter or changing the setting on it and see if performance changes.

Antony it sounds like classic limp mode. See the writeup in my signature.

Sorted
After scanning with VAG COM....nada, nothing, zip. everything came clear
The mechanic suggested to try and use:
BG 248 Maximum Diesel Performance. One can 11oz in the diesel tank, topped off with diesel and went for a trip....250 miles. In the first 3 to 5 miles the boost cut off twice so I have to stop and restart the engine ......after that....The car was flying, instant boost, much quicker acceleration...it is a miracle.
The BG248 cost $16.21 including taxes. This is what it does:
From the web site.

BG 248
Maximum Diesel Performance
BG 248 cleans the entire diesel fuel system including injectors and nozzles to restore maximum fuel atomization. Increases the cetane quality of diesel fuel. Contains BG Diesel Pump Lubricant to protect pump parts from damage or failure due to low-sulfur fuel. Special pour depressant additives isolate wax crystals as they are formed during cold weather and prevent their growing together; thus, fuel flows freely at sub-zero temperatures. BG 248 reduces exhaust smoke, provides smoother and quieter operation, improves fuel economy and restores maximum power and performance to diesel engines. Catalytic converter and oxygen sensor safe. This diesel fuel additive complies with the Federal low-sulfur content requirements for use in diesel motor vehicles.

When the mechanic told me to try it I was skeptical, he told me that it will clean my intake and turbo as well. He recommended that I use one can of BG248 every 4 or 5 tanks of diesel.
I don't know if this resolved my issue but oh boy, the way how the car run is unbelievable. The engine noise is gone, it sound so smooth now...

Where to get 248 BD product

This is a pretty old post but, if anyone is still checking here, I would like to know were your got the BD 248 for your clean up.
From your info it looks like you are in the SLC area..any help would be great as I'd like to try it on my TDI with rather poor mpg's.

Thanks for checking

Hey an60an, how about posting some photos or videos of it? I'm sure we can easily answer your question.