I used to have an older Buick Park Ave 99, very nice riding car, ran well. Any time I worked on something electrical and had to unhook the battery, it would send a spike through the system upon rehooking, that would almost always shock the tach into going all the way to the right side. Once my tach got to halfway, it automatically went all the way to the right, meaning it only needed a spark to get just past the halfway point, and it would go all the way due to the way it is springed inside. The first time I removed the cluster and wiggled a small tool in there to push the tach arm back to left, and once I got it just past halfway, it sprung back to the left like normal. Well, this happened a lot, so needless to say, I drilled a small hole in the cluster glass just above the center of the tach, barely visible. SO every time it did it, I could just stick a small tool in and wind it back to left. Anyway, some fixes for this could be a failing battery or alt, or bad grounds, or bcm.