Well, the crank sensor is used for engine speed not vehicle speed. The sensor you should be concerned with is in the differential housing and it is the vehicle speed sensor. Your best bet for diagnosing it is probably going to be in getting it to duplicate and then have an oscilloscope hooked up to the sensor and see what it says.

Why it shoots up to 70 and sticks is anybody's guess. I would fault trace the sender first and if it checks out ok while the problem is duplicating and all of your power and grounds are ok at the speedo you will probably need a new speedo head to fix it. I have occassionally found problems in the printed circuit cards on the instrument clusters but they usually don't show up as an intermittent problem.

Mark