2002 C70 Erratic Turbo Boost Development

[ryanvolvo]

My second post here and looking for some tips, suggestions and opinions regarding this current issue.

I'm noticing erratic turbo boost development. In second gear, boost builds up steady (although I probably don't notice the issue because it builds so quickly). However, in higher gears, the boost pressure "flutters" - it drops and builds again. It's being dumped instead of holding on to the pressure.

When driving the car, it feels like it speeds up then levels out, then speeds up, then levels out (pulsing acceleration)... etc.

Please see the attached image with a graph showing the boost pressure under acceleration.

What should I be checking? What could cause this "dumpage"?

Note: in an attempt to fix this issue, I've installed the iPD Heavy Duty Turbo Control Valve and also installed an uprated compressor bypass valve with stronger spring, "The included blue spring is 10 lbs and good for up to 17 lbs of boost." --- These fixes did not resolve the issue.