never had this issue but would ask if there possibility of getting a bad TCV from IPD? Also, are you 100% the rubber hose routing is correct :)

i would also start by maybe pulling the TCM and ECM modules, disconnecting battery power before doing so. Let the car reset itself if you will and try again, perhaps there is gremlin in the system that only a hard reset may clear out...

Hunting at 45, if I remember correctly these cars should go into 4th lockup around 47-48 MPH. I'd imagine the lack of boost, paired with the TPS input may be freaking the TCM out from operating as it should and going to lockup.

Again, been a while since I've been under the hood of an 850....but on my S60 there is also a manifold pressure sensor (drive my wire engine) that comes off the intake, meets up at the air box and loops into the turbo rubber hose matrix. Regretfully I cannot even speak to it in full detail as I've had zero issue (knock on wood) with this car and have not had to dive into the turbo plumbing. Understanding the TCV works in conjunction with the turbo actuator, are there any other pressure sensors on the manifold side of the older 5 cylinders? If so you may want to look there?

Steve