My speedometer and rev counter started to fail intermittently in my 2003 2.6 saloon.
This topic has been reported before and their replacement was recommended. I managed temporarily by using a sat nav as that that gave me my speed and made driving legal.
During the lockdown the battery started losing its charge. I replaced it and now these instruments function without problem.
I think (and hope) it must have been a problem with the battery voltage dropping fractionally, sufficient to stop these instruments working but not enough to affect starting etc.
The car has done 154,000 miles using no oil and even the air conditioning works thanks to the previous owner, an engineer, who replaced all the air conditioning seals and maintained the car himself. It has heavy duty rear springs so I do not have the problem of it bottoming on the suspension when loaded and these have only a minor effect on the handling when unloaded.