It sounds like the speedo itself as RS has seen.
Those are the symptoms - trips reset and no sweep; it is not external to the cluster and replacing it will cure, confirming is internal issue.
I've asked previously for someone to send me their failed cluster so I can at least take a look to see if I can find something (no guarantee I can of course), but no takers so far.
(not practical for you to send from UK)
The speed error is interesting however, have not seen that reported previously as an associated issue.
If you reset that error, does it come back?
It is possible it may be the input power for the sensor (which originates from the cluster) causing the problem and that would be interesting if so, since another 'power' related issue
- clocks sweep, trips and the power out to the speedo are also power rail related vs a signal issue, so pursuing something on those lines is what I would do if I can look at one.
Not sure how comfortable you are in breaking into to it or even what to look for when you get in there; so your practical solution is going to be replacement.
Your concern about possibly being key-switch related would certainly not be the cause here.
Key-Switch has its own problems, but that cannot cause the instrument issues you are seeing.
