intermittent boost problem

[Brian C. Mills]

I have been trying to track down a boost problem on my 200 20v sedan for a
while now.  I am fairly new to this so please excuse any mistakes I make in
providing the appropriate details.

I have 146K on the clock with original turbo, manifold, exhaust.

Stage III+ Chipped ECU from Intended Acceleration
Samco Silicone Boost Hoses
TurboXS Bypass valve

All of these mods were done by the previous owner.

I have driven a few other 200 20v (both sedan and avant) with similar
configurations, so I have a point of comparison when I claim to be having
issues.

Symptoms:

Boost will cut out and surge back.  For example, in 3rd gear WOT, the stock
boost gauge will briefly hit 1.8 and cut out completely to 1.0 or less and
then return only to cut out again.

The frustrating part is sometimes this won't happen, so I have been told by
"audi mechanics" in the past that I have no problems.

The check engine light is on.  If the fault codes are read it may list one
of the following codes:

Internal ECU Fault
Engine Speed Sensor

I also have a stock ECU, when the stock ECU is in no fault codes are read.
I have been told that the Internal ECU fault is common on modified ECUs and
should be ignored, should it?

I have checked for obvious signs of boost leaks and found none.  Eventually
took it to a tuner and had it "smoked", they confirmed there are no boost
leaks in the system.  The tuner put the car on a dyno and found:

"The car made about 193 hp to the wheels and a 210 lb torque to the wheels.
However, the boost is being electronically cut at about 5300 rpm."

Their most recent guess is that something is wrong with the wastegate.  I
have taken a look over the troubleshooting list at
http://www.sjmautotechnik.com/trouble_shooting/20vboost.html but I am hoping
someone here will be able to narrow the list down a bit.

Thanks for your time and sorry for the long post.  Please feel free to post
suggestions off list if you think this topic has been too well covered in
the past.

Brian C. Mills