Oil temp gauge on 512tr inop - BTDT

Having posted this question once before and researched the archives without success, James at Norwood found the answer on my 512tr. First he replaced the sensor having tested the gauge but aht "no work"; then he tested wire continuity but that "no work" - then discovered that the ground for this connection is through the bolts that hold the oil tank to the space frame. For some reason these bolts were not making a reliable ground as the car warmed up. Logically that might happen if theywere loose and the engine was cold but this is the reverse. Problem was solved by a ground strap, but on the next belt change this tank my offer a clue as to how this fault engineered itself.