I fixed in my case by checking all the connection (740wagon) seems that problem was with loose cables right at the connection with sending unit and also dirtyand corroded connetors in the compartment above driver side fender (that is in 740 wagon). Using simple multimeter you can actually test entire circut from inside the car, one method by checking resistivity or by removing plugh from sending unit and "shoretening it" and using a battery checking if you have current. For these operations you must remove instrumental panel and locate speedo cables.

Hope this will help.