Tach jumps and then quits when warm

Hello,
89 928 from Japan
I hate german schematics
Ok so now that I have that off my chest, here is the problem.
The Tach works for about 15 minutes and as the temp in the car increases and the dash gets warm, the tach quits.
I can reach under the dash and squeeze what I though was the cluster but now that I have the dash out I am thinking that it was the signal, washer, dimmer arm assembly.
To me it seems like a loose connection that is overheating, so I pulled the dash and started looking through the shop manuals for the correct circuit.
Thats the problem, I can't find it in the manuals.
anyone able to give me some direction.
of coarse I am going to check all the pin terminals and the tach connection on the board.

Thanks in advance

This is purely a guess...

It looks possible that the engine speed signal comes from the injection system. Near the top of the instrument cluster block on Page 97-317, there is an entry on Plug 4 Terminal 8 for "L-Jetronic". A yellow wire leads to J12 going into the Central Electric Panel , thru the Panel and out on W12, where a brown/red wire leads to a flag in Zone N26 containing "L89". In Zone L89, there is a flag containing "N26", where a brown/red wire leads to a box containing "2". This box represents a hard connection in a wire harness that feeds half of the injectors. When the L-Jetronic fires the injectors, the circuitry in the instrument cluster reads the pulses, which gives the engine RPM for the tach.

That being said, I think that the problem is most likely in the connections on or in the instrument cluster...

Thank you Wally,
I looked at my Tec-4 manual and see that I connected the tach wire to the brown/red wire on the W12 centre
panel connector. so i started there by connecting one lead of of my Ohm meter.
i used the other end on the meter and started checking for continuity at each pin on the cluster connectors which i found at a yellow wire on one of those plugs
upon inspection of the plug i noticed the pin grip was uneven so with a needle size tool i flexed them all tighter.
(NOTE: tightening to much means you fight with reconnecting the plugs)
This seems to have fixed my intermittent tach signal.