C 5 Rev Counter Failure

[Guest Brian]

2001 C5 2.2 HDI. Rev counter gradually failed over about 500 kms. While cruising at about 2500 revs reading dropped to approx 1000. Next time driven it read about 500 max. Gradually each time I drove it dropped to 0 revs. Sometimes when I start engine the needle gives a little kick but returns to 0. No fault showing on computer screen. Where do I start? Electrical or mechanical fault?

Would like some ideas or advice before resorting to Citroen Dealership. Anyone with same problem?

Car is based in Spain and is L.H.D. 170000 kms on clock.