12-6-8 boost patern
 

Stock car, only a downpipe. Vacuum reads a little low at idle, but that's another situation.

Boost spools very fast on the primary and actually gets up to 12psi, transitions drops way down and never recovers. If I do a 2nd gear pull I get 12psi, 3rd gear from 2500RPM and it will spool up to 10.5-11psi and hold until transition, but drops to 6 and then 8 on the secondary.

Replaced the inlet hoses and checked everything to insure nothing is leaking, verified on a 2nd boost gauge.
I do have oil in the intake hoses, more than I'm used to seeing, but it's also hard to tell how old the oil could be.

Any indication of when this started happening (ie did you just service something, or just buy the car, or was it random)?

I had a similar issue - 6 psi or so on the secondary. Turns out my pre-control and wastegate solenoids plugs were swapped over.
Pull off the little black vacuum chamber and have a good look around for any popped hoses or split check valves.

I just bought the car, so it's hard to know how long it's been going on for. I'll look into the vacuum chamber and see what I can find.

check the turbo control pressure and vacuum solenoids. also precontrol and wastegate solenoids and make sure they are plug correctly like Enervation mentioned.