That's got to be an electrical fault. There's no way the engine temperature can actually be going from 95 to off the scale in half a second, then back down to 95 a few seconds later. Assuming the thing doesn't boil and throw all its water out when this happens (i.e. it's not actually overheating) I'd probably start by disconnecting the temperature sender and seeing if the gauge still goes off the scale. If it does, it could take a while to track the problem down. If it doesn't and stays stuck on zero, I'd change the temperature sender.