1985 745t m46

Okay, so I've posted with this problem a couple of times now, and I have yet to get a post with a sure solution (or one that works). That being the case, I figured I would post yet again with even more information in an attempt to find a resolution.

Over the last 6 months or so I have been experiencing a very odd problem with my brick. Basically from time to time the car will stumble horribly as soon as I enter boost (on the guage... whenever I pass the black and enter the yellow boost area). It feels like someone has suddenly cut off teh oxygen supply to my brick and it is struggling to boost but simply can't. In fact, the car actually begins to perform worse the MORE boost I give it, and has in fact backfired one time when I tried to "break through" the problem. This will usually happen for an entire trip (from the moment I start the car), and can sometimes be cured by stopping, shutting down, waiting, and restarting (though not always, and not the last time).

I originally thought it was a fuel issue, but I have replaced every fuel delivery component with the exception of the main pump itself (FPReg, FPRelay, Pre-pump, sender, etc.). I then thought it might be an exhaust issue, but the cat and exhaust manifold were both replaced by a top level indy volvo mechanic within the last 10 months (all volvo parts), and I trust his work.

My next thought was that this must be a sensor issue, and I'm currently in the middle of this phase now. I figured it might be a faulty knock sensor, faulty ETC sensor, or a fault o2 sensor. I have replaced the ETC sensor )which seemed to show possible fault), but there has been no change (it stumbled horribly on my most recent drive for 200 miles).

Could a bad o2 sensor cause such a problem ONLY under boost? I would think it would be apparent all the time... Would a sticky wastegate cause such a problem? I would think that if the wastegate were stuck open I wouldn't even be able to enter boost at all (since it would vent fully under all conditions...).

One last note. From time to time I purchase gas at a Coastal station that has very low prices for my area. Many folks buy gas here (it is prominently positioned), and I had figured they made money off bulk sales. While I lack the data to match my problems with fueling at this station, I DID experience my first serious stumbling in a while after I filled up at that station. Basically I entered with a few gallons left, filled up, and started stumbling about 35 miles after I left (I started and stopped in between, but about 25 miles all told). Is it possible that the station is dishing out bad gas, or that teh gas it is selling is not the indicated grade? I have had a similar problem when I tried to use fuel with a grade of 87 or lower in my turbo awhile back... it felt like the turbo was broken. If so, why would a lower octane ONLY cause a problem under boost?

Any information would be greatly appreciated. Thanks in advance for the help.

-rt