I had this problem with my speedo sender, i put it down to the diff flex/movement as this caused the Prop magnets being hit, a good solution is to get hold of the gearbox sender adapter from Burton Power, (not cheap though) but should sort out this problem. I got one a while back but as yet not got around to fitting it,
It may well be a tight fit on some cars with a narrow tunnel.
P.S If car driven like a grandad you should not have this problem, I'm a grandad and drive mine as if i am being chassed by the Grim Reeper!