After fiddling around on my first boat with tacho repairs I replaced them with VDO truck ones. A fraction of the price of the volvo ones and the same diameter. My repair guy told me the circuitboards get damaged by the pounding and vibration. He told me the one VDO made for volvo was less robust than VDO's own.
I had a similar problem with one of my VP tacho's and after fault tracing it turned out that one of the previous owners of my boat had moved the depth sounder to fit a plotter in the dash and hadn't earthed it very well. The subsiquent interference had mean't that one of the tacho's wouldn't read properly or hour count for I estimate 10 years judging by the difference in hours registered. After I rewired it properly it works perfectly.
It may be something simple like that. BTW they are VDO guages rebadged.
Thanks for the info
I have made contact with speedycables who have quoted me £80each plus VAT- Much better than nearly £250 each new!
I will let you know what the service is like (and if they fix it!)