I have a stock 1998 Dyna Convertible and one day the tach needle just started jumping up 8,000 while sitting at a stop light. Now the needle jumps around most of the time, at idle or while riding. I have the 6,000 rpm Screamin Eagle ECM, but it ran fine for years, and now just all of a sudden. Is there a way to test without the $500 speedo tester? Bench test on the tach?