If the rev counter is dropping its not the fuel pump.

Working backwards from the counter:

rev counter gets fed by
ecu output which is based on
the ecu seeing a crank sensor signal
which is based on the crank sensor working and the earths/positives being in good condition


So i would say either crank sensor fault, or earth probably.


Crank sensor faults are relatively common, and its good practice to change it every few years, so I would start with that on the basis it will do no harm to have a new one if the old one is all old and wire gone rigid etc now anyway. Buy from a trusted supplier to get a genuine one not a copy, MSD or Reyland or Burton etc for example