So I'm having some electrical gremlins. I've been browsing these forums for months now trying to get some clues but at this point I'd like to ask for help directly.
The car is a pre-facelift CLK320 (1999)

Issue: When the external temp and car/cabin is cold (so up to around 10C or 50 F) and I start the car I get the display defective message on the instrument cluster with no more info or pictogram to help me. In parallel with this every warning light flashes erratically and all 3 gauge needles are jumping to zero then back up to the value then back again. Other than this both the engine and gearbox works perfectly. Comfort electronics such as power window, seat adjustment, seat heating, etc all working correctly except for the cruise control. It turns on but sometimes turns itself off.

Once the engine/cabin gets warmer the flashing lights and jumping needles go away and things seem to be normal but then....
Then usually when the engine temp reaches 60C, the radiator fan starts up and it desperately tries to keep the engine temp. at 60C. The other thing that used to happen is that the temp. goes up to around 80C and all of a sudden the radiator fan comes on at full blast and never stops. Last time it pushed down coolant temp. to around 45C.

The short term solution is that I restart the car. When I do that, everything is normal. The fan is keeping coolant temp. around 90-95. Not even display defective message and the cruise control also works.
So my issue is 100% engine/cabin temp. related.


What I already tried based on my research:
- Checked and re-seated all fuses (in engine bay, side of dashboard and in the trunk)
- I took the battery to a battery shop to check it --> battery in perfect condition and the proper size (100 Ah)
- Checked if there is an electric parasite drain in the car --> No (also have good voltage before start up)
- measured alternator voltage on idling engine at the battery --> OK
- re-seated the instrument cluster sockets (no corrosion on the pins)
- checked ground wires (engine bay and behind instrument cluster) and restored connections with sandpaper
- checked SAM for rust on PCB --> it was shiny, no signs of water intrusion and solder joints were not cracked
- checked K40 relay (not just the square relay but the whole relay block) --> it was shiny, no signs of water intrusion and solder joints were not cracked
- checked and re-seated the ECU connectors (no corrosion on the pins)
- replaced coolant temperature sensor

Unfortunately none of the above made any difference and I'm absolutely out of ideas... and hope..
I have an adapter for the 38 pin OBD connector but the issue is that this way I can only do a basic engine diagnosis but I have no error codes whatsoever so I guess the problem is in another system.

Of course I could buy a used instrument cluster but I don't really want to throw money at the problem blindly. I also don't see how a faulty instrument cluster could make the radiator fan come on in such an erratic manner.

Can you guys please share some wisdom with me?