I was driving the truck the other day and truck was running fine and noticed the tach needle jumping around like crazy throughout the rpms ... The truck runs fine and doesn't idle rough or anything, just one min it was fine the next its bouncing around. It's not steady.. I have one of them aftermarket sun pro tachs cause the stock one in the guage cluster never worked right. Checked all my connections and nothing came loose and no wires are broken. I checked the coil and its good. Checked the ignitor or icm and its good.. any ideas as to what's wrong?