I had exactly the same with my Vectra, I was driving along and the speedo and rev counter just stopped working, pulled over, turned off the ignition and started again, all was fine and has been since, just a glitch
I can understand your frustration. Its typical of software driven electronics. Could be one or a combination of several things.
Unless there is a clear wiring problem or common fault I suspect you're on a hiding to nothing to keep looking for the problem.
These kind of things can be massively difficult (and frustrating) problems to isolate and fix - and its not just cars. We normally isolate and fix them by swapping out systems one by one - something can't do in a car easily and definitely not cheaply. Simply disturbing wiring and connections can quite often be sufficient to drive the problem away for a while at least.
Occasionally the software might throw a glitch and not set up correctly on initialisation, or even simply stop working, especially in older systems with old degraded components that aren't quite within spec or with slightly dodgy connections, especially to ground. It might be the software has detected a minor problem and gone into a safe mode, or simply think things have been switched off and only start working again if the expected start-up cycle has been initiated. All software driven systems can do this . Restart (just like PCs! - Turn Off and Turn On - TOTO) and everything gets reset and tries again. More often than not the trigger for the problem has gone away.
Might also be a problem component in the electronics, a capacitor or transistor component. Restart and they get biased slightly differently and kick into life. Also applies to connectors - they corrode and while they look OK, the connection point is degraded until it forms a diode type effect. Move it and it either fails or resets the connection. Only if they fail permanently are you likely to isolate it.
Low voltage low current situations are worst as you don't get any contact wetting on connections. These kind of faults tend to be more persistent though.
Got to remember the electronics are working in a very challenging environment and they age both with use and lack of use too. They go out of spec but don't fail. Might be quite some while before they fail completely, if ever.
Unless it keeps happening, I'd try not to worry about it. You've probably got rid of the problem (at least for a while) by looking for it.
