a shortwhile ago my tach stopped working it will go up to 14-1500 as the revs actually reach that and fall back to 500 then go up again but the revs don't fall to 500 they go up smoothly

sometimes u can start the car and the tach will work flawlessly for about a min and then repeat the current problem i am trying to figure out if the guage is shot or if the wire that sends the rpm signal is loose or shorted and how where the signal comes from i have heard the rpm signal comes from the coils