Yeah, it's fully electronic mate, I'm ignoring it for a couple of days though, it's a brain melter.
The entire display only comes on after maybe 10 minutes of driving, works sooner in sunny weather. Takes longer in cold weather.
I keep coming back to resistance (as far as I'm aware temperature can affect resistance).....pickled.
I think you're on the right lines. A common method of fault finding is to use freezer spray on components or tracks. If there is a fault, it is more likely to show up at extremes of temperature. The more I think about this, the more I come back to a poor connection. The connection is making or breaking under changes in temperature.
If it was only one part of the display affected, then maybe it could be a component in that part of the circuit, but from what I can tell, all of the display works either correctly, or not at all. This means that it is most likely to be something common to the whole display.
Without a circuit diagram, it is hard to know what could be common to the whole display, but it is likely to be the power. Now this could be a +12v feed, or there maybe some power conditioning in the display. This may be just smoothing or over-voltage protection, or there could be voltage regulation to bring everything down to +5v (common in digital electronics).
Of course the other side of the power that is common to everything is...... the earth!!
I think the power is also made prime suspect by the fact that the display resets every time.
So this brings us back to the same question, is it the display, or what feeds it. Knowing this will certainly help you out, but almost impossible to determine without a known good display to try.
If it is the display, I think you are in for a long hard ride to fault find it. The best you could hope for without circuit diagrams and test gear would be to closely examine every solder joint and re-flow any that look suspect, you may just get lucky. If it's a multi layer board and there is a crack in one of the tracks, then I'm afraid it's toast, as it will be almost impossible to find.
I really wish you luck with it and it sounds like you relish a challenge, but you may have to concede that this one will beat you.
I saw that phrase used once before where a "false earth" was created in a 18v circuit to generate a 12v DC potential (not automotive of course). It was achieved by using a 5.1v zener diode to bring the anode down by 12.9v from the feed, creating the +12v supply
Sorry for the off topic
There are certainly some pretty weird circuits out there!