I have a SW tach in my boat. probably 20 yrs old. It sat in the old one and worked fine off a mallory dist. It sat for some time and now in my new boat with HEI it jumps all over. Sometimes it will stay at 3500 rpm after the key is off and then comes back to zero after you click it on again. Other times while driving it jumps all over. and then sometimes seems to work just fine. I checked the ground and it seems pretty solid. I know if you have the wrong cylinder setting it can cause this but it worked well in the old boat with a v-8. I also get inconsistant readings on my fuel guages. sometimes they are accurate and other times way off. Im not sure if my entire system is out of whack or what.