First off, to answer your questions, anything in terms of electronics/computers can act up intermittently.
It sounds to me like a fuel mixture issue, given that when I had a nasty intake leak (read fuel mixture issue) it would do the same thing: rev up fine to almost 3K if given steady pressure, but bog down if punched or pushed past a certain point.
Given that it comes and goes, I would agree with the ECU issues.
You said you got all the turbo hoses, and I wanted to point out the obvious and ask about the short 4-incher from the intercooler to the metal intake tube. It's easy to overlook, especially if someone else replaces "them all" for you.
This is a stretch too, but I'm trying to be creative - what about the O2 sensor? Ever replaced? I don't see how it would work intermittently though.

Good Luck,
Will
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1990 740 Turbo, on its way to stock specs, maybe beyond