Troublesome factory tach
 

After letting the car sit for several days then going out to drive it the tach occasionally doesn't move off zero until starting to drive down the road and getting above 1500rpms. Other times (not often) it can be about 100rpms off highway cruising speeds. (60mph is around 2K but it will read btwn 1800 and 1900.) Anyone know how to fix or at least where to look besides getting an aftermarket tach. Thanks.