I'll probably test the wastegate opening pressure first. It may be fine (and I suspect it is), in which case there is another problem, that I'm hoping someone on here can point me toward.

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1986 300 SDL, gray on black, #14 head, 180K miles
1982 300 CD, silver on palomino, 290K+ miles [sold]
1980 300 SD, white on palomino, 500K+ miles when retired [uncle's car - inspiration for the above]