Is there a sender, sensor, or other part that could be the culprit rather than the gauge itself?

I have a 1977 911. I had the VDO speedometer professionally repaired a few months ago b/c the odometer and speedometer both stopped working intermittently at the same time. Unfortunately, the same problem is happening again. The RPM gauge is working fine. Is there a sender, sensor, or other part that could be the culprit rather than the gauge itself?

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[Casey at Pelican Parts]

Could be the speed sensor at the gearbox. I have also seen oil contaminate the connection near the gearbox mount that caused sporadic speedometer and odometer operation. 911-606-209-00-M100
http://www.pelicanparts.com/cgi-bin/ksearch/pel_search_2014.cgi?SUPERCAT_FLAG=Y&make=POR&Context_make=&please_wait=N&LastVisited_input=0787&Previous_Section=ELSWCH&forumid=&threadid=&command=DWsearch&description=speedometer&I1.x=0&I1.y=0 ] http://www.pelicanparts.com/cgi-bin/ksearch/pel_search_2014.cgi?SUPERCAT_FLAG=Y&make=POR&Context_make=&please_wait=N&LastVisited_input=0787&Previous_Section=ELSWCH&forumid=&threadid=&command=DWsearch&description=speedometer&I1.x=0&I1.y=0 [/url]

- Casey