VECTRA SPEEDO & REV COUNTER NOT WORKING?

We have a 98 S VECTRA 1800 ARTIC PETROL SALOON. The speedo and rev counter are not working, sometime they are fine other times nothing, when you switch the engine off, the needles flicker etc. We have tried a second hand unit the speedo and rev counter worked but nothing else IE the indicator warning light became the main beam blue light and so on, so we have reverted to the original unit, mileage etc is fine, the alternator is charging at 14 is it ohms? Not sure, but it isnt over charging, battery sems ok, any ideas i have read it could be the ABS sensor for the mph, but what about the rev counter?.