Tach not working right.

Sometimes the tach in my gauge cluster will not work right. Like when I start the car it just sits their stuck but as I start to drive the tach starts to go up very slowly. The longer and harder I drive the more the needle moves, the needle will reach like 1200-1500 and then break free and start to work normally. What is messed is up is that on some days the tach works fine stright from the get go. Any ideas on what could be causing this?

Check the connection at the coil, my first guess.... Could be loose or corrosion...:)

Well, I live in a humid climate and the tach has problems in the mourning and at night. So I'm guessing somehow moisture is getting into the electronics from some where.