Error 1237 - Fuel Shut Off Valve - intermittent error / N109

Volkswagen Golf 3 '97 TDI 1.9 / 1H1.
Error: The engine simply dies; sometimes it restarts afterward, but other times it takes several minutes to start.

Error codes read: see the title of this thread.

So far, I have completed the following:
- Disconnected and inspected the engine control unit connector; it looks great, no corrosion; reattached.
- Relay N109 replaced with a new part.
- same with the fuel shutoff valve.
-I once connected a control light in parallel with the shut-off valve to ground; it's definitely clear that the shut-off valve deactivates (light goes out) when the engine stops.
-I've now connected a switch in parallel with the solenoid valve to +12V (with a fuse), so that I can keep the valve open; a short test run earlier looked pretty good.

Do you have any suggestions for other places I could look?
Is it worth removing the main circuit board located in the footwell, which contains the relay sockets, and inspecting it for broken traces, cold solder joints, etc.?
Is there any known issue with cable breakage in this case?

Hi there,

I experienced hard starting issues and sometimes the engine wouldn't start at all: The starter would turn, but nothing else happened.
I had cold solder joints in the control unit of the alarm system. There are three relays (or what I believe to be relays) on the circuit board, and I resoldered them. After that, the problem was resolved.

Regards,
A.

My car doesn't have an alarm system or immobilizer. I'm considering re-examining the relay board and possibly resoldering some connections. However, that would be quite a bit of work. That's why I wanted to hear if anyone else has had similar experiences that confirm or refute this as a possible cause of the problem.
Probably a broken cable somewhere. It's really annoying to find.
Well, the "emergency" switch to open the fuel shut-off valve seems to be working. So at least I won't get stranded in a white snowy landscape.