Good evening. First time posting but I've mined lots of good data in the past. I have an issue that has been ******* my car for a while and tips would be appreciated.

Car has 213k on it and had a CEL showing p0134 and another code I can't recall (the two were for O2 sensor 1 and catalytic converter.) I'd been chasing vacuum leaks and had fixed a few but these kept coming back. Eventually symptoms were wild downshifting and no power so I took it to the shop for a diagnosis. They found the turbo was shot and replaced it, ouch but not a surprise. Car ran great for a day or two but then redeveloped the downshifting to a lesser degree. Took it back and they had it for a day, smoke tested it and found the likely suspect was the boost control valve. Car ran well for several days but now my perception is its not running that strong again. I'd like to mess with it more myself now (I'm cheaper by the hour) but I'd like some tips on where to look.

Ohm test the N75?
Pump the system up with 10 PSI again?
-if I do this there should be zero air leaking right?

Thank you for any help you can provide.