The tacho is a simple device with an internal magnet, it's possible that something has been attracted to it and a 'slap' dislodges it temporally. It could be a flake of rust from the inside of the base. The only way that can be fixed is to take it apart to see what the cause is. A replica tacho is available but from what I've read on this forum the quality of them isn't great, basically they are a copy of a Nippon Seiki gauge and not a Nippon Denso gauge originally fitted.