If you are running E3 sparkplugs your tach will start to bounce about 3 days after you install them. They are the biggest overpriced piles of crap on the planet.
I was actually temted by the E3,if only because of the good luck I had with splitfires in the past. They stayed so clean you could never read the plug as a tuning aid!
They are pricey though. I'll wait till the're on sale and try em in the Van.
I had the tach directly grounded to the battery negative. Maybe I'll ground it elsewhere,or tap into the line and add an additional ground somewhere in the interior.