You are wasting your time looking for the problem anywhere in your main electrical system, including it's grounding points.
The only circuit that matters is the self-contained circuit between gauge and thermocouple. This circuit is very low voltage, and so it is very sensititve to imperfect connections. It takes only a small bit of surface corrosion or contaminants between the connection lugs to break the function of the gauge.
I'll bet it's either merely coincidence your pyrometer started going haywire when the clutch was installed, or it is because the wiring was bumped or strained during the clutch work (IE: they removed the transaxle to access the clutch and tilted the engine down which strained the wiring, or they bumped the wiring during the work). This could have upset a weak electrical connection or moved it a little bit so it is no longer a perfect (or only an intermittently perfect as a result of heat, vibration, and/or engine movement) electrical connection. The pyrometer/thermocouple wiring also has a solid non-stranded wire in it, which is much easier to break from fatigue than standard stranded automotive wiring is.
You might want to also try the tips I posted in this thread:
http://www.vwdiesel.net/phpBB/viewtopic.php?t=687Hope this helps... Good luck.