The day after I got my IR, the local area (mountainous) was socked in. I launched myself in a rental to go practice IFR. Well, the plastic static tubing behind the panel had a crack in it...everytime I hit a bump the tube flexed and allowed cabin ambient pressure into the static system causing my altimeter to bounce all over the place. Since I had no idea if it was even working at all (it was, just within a range of several hundred feet) I become very concerned about the local rocks...got vectors into the clear and stayed there until I could land. It was prett scary for a few minutes.