I'm hoping someone else will have run across this problem and found a good fix. I did a search, and couldn't find another topic that really matched mine. In my '90 Sierra, the speedo will occasionally just drop down to zero while I'm driving. If I reach up and thump the instrument panel lens (sometimes takes several thumps) the speedo will come right back up. I thought the problem was in the speedo itself, so got one form Pick-N-Pull and swapped it in, no change in problem. Pulled the cluster to check connections, all the connections were clean and appeared solid. That was as far as I could go, I'm no electronics tech, and I wasn't about to start getting into the diodes, resistors, and computer chips of the printed circuit board. Any suggestions out there?