Awhile back my Tach did the same thing. On suggestion of some DucatiMS members I took it out and tightened 2 little screws in the back with a small flat head screwdriver and it started working again. You might try it before you spend a load of cash
Its been awhile since I had to fix mine so.... If my memory serve me ight there were 2 brass flat head screw that were horizontally paralel to each other (WOW that was technical) those where the 2 I tightened up. I think there might have been a 3rd on above those 2 Im not sure!? I see If I can find the old post in archives.
Hope this help