my bet is EGR is sticking or is stuck just a bit .
on start up you can tell right away if teh vgt vanes are stuck
stand at the tailpipe have someone start it up.
at first catch it should sound like a big block no pipes, deep raspy
then it will soften back to a purr as the vgt vanes are moved by oil pressure to the idle position
if the cat was choked up... you would have higher egts and less power per normal psi of boost
egr is a quick two bolts 5/16 (8 mm) and pop her out and see just how nasty it is. mine has been fluttering and yep each time it does it i can smoke the folks out. followed by low boost and a check engine light
followed the dts sites instructions on cleaning it and so far so good
egr cleaning