[T3] a stumper!

[Phil Hof]

Radio interference might be possible as a cause.  Perhaps at 3000 RPM the spark is causing a circuit resonance that unlinks at lower or higher RPMs. VW added a bracket to keep the plug wires away from the FI leads for a similar reason:
https://www.thesamba.com/vw/classifieds/detail.php?id=2421190 <https://www.thesamba.com/vw/classifieds/detail.php?id=2421190>
Right side of this photo:
9027914.jpg <https://www.thesamba.com/vw/classifieds/pix/9027914.jpg>
The bracket needs some plastic HT lead holders, which are still available, too.
I think the Type 4s had the same issue.
So I’d make sure the HT leads are away from any FI wiring as much as possible.
Yeah, not too likely, but you’ve already eliminated the common causes!
-Phil
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