91 200q stalls/loses boost

[Sargent Schutt <sargent@symposiafoundation.org>]

Doctor, doctor, gimme the news...

Thanks to some ham-fisted mechanics (no, not me this time) my car (91
200q - TAP chip/spring, etc) now stalls after being pushed hard - e.g.
you stand on it to make a light, as you approach the light to take a
corner or stop, clutch in, the motor will allow itself to stall. 

The problem arose after replacing the vacuum line (had small leak) that
runs from the turbo cut-off valve to somewhere behind(?) the engine. 

The funny thing is, the check engine light will come on before it
actually stalls - so probably not the idle stabilizer. Sometimes the ISV
will compensate, sometimes it can’t. I also checked/replaced the turbo
cut-off valve and that is not the problem. There are no evident vacuum
leaks, though the problem would seem to be in the vacuum system. 

Also, Part II of this problem (probably related) is a loss of boost
after 5000 RPMs. It will make it to 1.9 bar, then fall off to 1.8, 1.7
sometimes 1.6 as redline is approached. The turbo cutoff switch
registers fine. I’ve seen readings as high as 2.0 - just prior to taking
it in for the repair!

Part III - also related I think, throttle response at low RPM in 1st/2nd
feels more sluggish than before, as though the engine is running rich.

Might this be electrical (chip?) or vacuum related? It would seem that
it would be vacuum related as the problem arose out of the replacement
of the aforementioned vac hose. 

Please help, this stalling stuff has got to end! (not to mention it
should be going faster!!)

TIA!

Sarge
91 200q (sputtermobile)
86 5ktq (laughs at sputtermobile with throaty Borla grin)