Which sending unit? What kind of ignition system do you have?
The first suspect is the ground strap between the engine and frame, the battery and frame, and the dash and the battery. The voltage regulator references to the engine ground if it is an alternator, or the dash ground if it is a generator, so if the ground strap to the battery is too resistive the voltage that the sending unit sees between gnd and the controller (gauge, ignition, whatever) is too high and it can burn out. Also, it could be seeing RF noise which puts little high voltage spikes on it. Both could do it.
Another enemy is heat, but usually that's not a problem on a 6 cylinder distributor.
