2004 w/ 118k miles, recently had the timing belt replaced (water pump, radiator, spark plugs) now the tach will sit at a base line showing 800 rpm when engine is off. Once running,the rpm will consistently reflect the extra 800 rpm. This is intermittent so I can't determine any other coincidental findings (electrical load etc). Any ideas? When the tach baseline is 0 it works correctly but can revert to the error while driving.