i've just mounted a blow off valve to my 850 t5, it's an automatic and when i step on the gas it goes like normal until it changes up a gear. after it changes gear it starts to jerk like hell, maybe worth mentioning that i have a fault code 2-3-1 (Long term fuel mixture too lean or too rich in part load stage) and i suspect a vacuum leak from the pcv system. also after i installed the bov i gained quite a bit off boost pressure.

 

The BOV has nothing to do with your problem. The vacuum leak does. When your car shifts gears it loads up the engine more and your boost spikes. The ECU attempts to control it and is probably sending a signal the the wastegate to open but it cannot so you end up with higher boost. The car gets up and goes better because of the boost but at the same time you hit the 14.7psi fuel cut off point and the fuel is shut off for a second making the car fall on its face.
You have a vacuum/boost line issue in the wastegate line.