Speedometer intermittent
 

Every so often I look down at the speedometer while driving and the needle is on zero. After fifteen to thirty seconds it shoots up and settles back in to where it should be. Does the series III use a cable driven system, or does it use some type of sending unit?

Until '83 or so a cable system was used. After that Jaguar went to an electric speedo using a signal generator....usually called a "transducer". It's on the right side of the trans, towards the rear. It's almost always the culprit when the speedo starts doing strange things.

Cheers
DD

Wow! Those transducers are spendy little critters.