Oh, silly me. That will teach me not to reply at the end of the day just before going home. Your tach does return to zero when off, so it is not sticking.

Then it would be the internal circuitry. If you are electronically inclined then you would know what an electrolitic capacitor is. They are the first components to age, and replacing them will possibly fix the problem. Alternatively, the solder has cracked due to age and all the components are not connected any more.

Cracked solder is very common with newer Volvos (a Bosch problem) due to the manunufacturing processes. On the 240 and 740 it shows up as a failed automatic overdrive relay or fuel pump relay.

Have fun...