HELP! Tachometer's gone psycho!

My tachometer worked fine until a few weeks ago. Now the needle bounces all over the place with no correlation to the engine speed. :wack: I have a '71 340 with a Mopar Performance Electronic ignition and a chrome ECU. The tachometer is a Stewart-Warner Deluxe Series 0-8,000rpm. Does anyone have any ideas how to troubleshoot this?
:banghead: