[s-cars] Baffled over boost

[Mock, Bruce J (Bruce)]

Hi Guys:
 
This list is truly priceless.  I've been on this list almost since its humble beginning and have soaked-up immeasurable bits of wisdom to the point where I feel like NO matter what goes wrong with my '93 S4,  (and at 175,000mi it really has not been much at all! )  I can tackle the problem.  I know most all of us would agree about the great worth of this virtual community.  Thanks guys.
 
This little problem has me completely stumped:
 
Again:  '93 S4 w/ 175,000mi stock expect for MTM stage 1 chip and RS2 Ex. manifold.
 
Car runs fine until boost builds past ~15psig (I have an Autometer gauge on the A-pillar)  Normally boost would continue to build - depending upon what gear I am in - to ~20-21psig.  But now, at approx. 15psig, instead of the boost building - the car goes Drrrrrrrrrrrr.  (technical term, I know)  Seriously, It sounds like a toned-down, quieted-down version of what a Diesel big-rig sounds like when they throw on the Jake-brake"  Drrrrrrrrrr.  If I keep my foot to the floor this behavior continues. Drrrrrrr.  If I pull my foot off the gas it stops and the car runs fine - BELOW that magic threshold.   Behaves this way in 2nd, 3rd, 4th and 5th gears.  Behavior is very consistent -whether the motor is hot or cold.  Again car runs, starts etc. just fine, no hint of anything wrong...  until you put your foot down hard and let the boost build up - no boost beyond ~15psig. Drrrrrrrr.
 
This has been going on for quite some time now.  I can't pinpoint when it actually started.  I went to to get on the highway one day - got on the gas hard- and this happened. It'd been like this ever since.  So... At least it's not an intermittent problem!
 
 
Here's what has been done to troubleshoot so far:
 
Tested for boot leaks by pressurizing the intake tract  - everything is tight.  As I increase pressure up past ~12psi the oil cap seal starts hissing slightly. (I have Samcos and and a high-dollar MTM Michelin-man hose)  I also inspected boost lines near the moisture trap, etc.
 
Multiple high-quality tanks of gas have been run through - so I don't believe that it is "bad gas"  Also, the fuel pump was recently changed due to a 'no start' problem.
 
Waste Gate Freq. Valve changed out for a known-good unit.  No change in behavior.
 
Near-new plugs changed-out for brand new ones.  No change in behavior.
 
MAF cleaned/inspected. No change.
 
Swapped-out the waste gate spring with a known good unit.  No change.
 
Disconnected the exhaust at the down pipe thinking that the CAT(s) was possibly clogged and restricting exhaust flow.  Man was THAT a loud test drive  :)   No change.
 
Throttle Pos Sensor tested per Bentley  (Resistance, etc. checks)  No change.
 
Wholesale swap-out of all 5 coil packs/spark plug boots/wiring - by this I mean I removed and swapped the entire aluminum cover section (the one that says "20V" and houses the coil-packs/boots, etc.) that sits on top of the valve cover with a unit from another car.  No change.
 
Swapped ECU with known good (stock chip) unit from another car.  No change.
 
 
With all the recent discussion re knock sensors, I will check to see if they both are tight.  The rear knock sensor looks to be a real bear to get to.  Any suggestions on how to get to this guy?
 
 
Well... there you have it.  I feel like I know my car pretty well thanks to this group.  But this one has me beat!
 
Appreciate any thoughts on where to go from here from the collective wisdom pool!!!
 
Bruce