Can someone explain how/why this happened ???

[Steve Cole]

So yesterday was a real scorcher, about mid 90's most of the day and 40% humidity. My plan for the day was to drive my truck to a local cruise-in but first we had to stop by the mall to pick up some new glasses for my wife. We locked the truck up and went inside for about an hour. Up til this point, everything was working fine. When we left the mall, we had driven about a 1/4 mile before I noticed my tach was on 0. We drove about 6 miles to the cruise-in and the tach needle never moved from 0. When I got there I shut the truck off and started checking/wiggling connections at the fuse box and back of the tach, all seemed tight so I started the truck and still nothing. Checked the connection at the HEI and started again, nothing. I stayed at the show about two hours then drove the 15 miles home, on 0 the whole time. I parked in the garage and started searching posts here on the board trying to figure out what happened. The only thing I could find was that if not properly grounded, the tach circuit board could get fried, and I also found a mention that the tach doesn't like heat, for whatever reason. Anyway, the truck sat for about thirty minutes during this and after reading several posts, I thought my newly installed OEM tach was fried. I went out to the garage and started the truck and low and behold, the tach was working! ??? Relieved, I made and installed an additional ground wire on the back of the tach.

My question is, how could it read zero for so long then start working again? I thought it was a goner for sure! Thanks,