I have an 806 which may have had a similar fault. What exactly is happening? Did it stop all of a sudden? Does it soemtimes work but not read correct speed?
The problem may be the drive housing on the rear of the spindle to which the needle is pushed on. It houses 2 small gears kept in place a tiny plastic lug which breaks off causing the gears to stick. There are several places that will repair tme but not cheaply. If you can get the instrument pod out then there is a place in Holland that will mail the part to fix it (new housing). It's a fiddly repair but straight forward to do.
Tachoshop.eu is the the place and you can email Maurice on info@tachshop.eu. He is very helpful. Otherwise BBA-Reman or KC Systems will quote for a repair but they are not cheap considering what is involved. BBA quoted £150 + VAT and shipping.
Hope that helps.
Barry
February 2009