Don't know what it is, but have you double-checked all of the wiring, where you can?
What I'm thinking of, as this problem is going on for you for a long time, is that maybe somewhere there is a frayed cut or damaged wire(s).
Just a suggestion.
I presume you've pressure tested all of the pipes, vacuum and boost including the intercooler ?
If you ever find the problem before you burn it, please do post the cause of the heartache?
Wishing you maximum luck with it.
Get a drive / lift in someone elses, just to keep your faith in tact.
Kind regards.

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