'94 Dodge Spirit- bouncing speedometer needle

I have a 1994 Dodge Spirit, with an automatic transmission. Sometimes, but not always, and usually in cold weather, the speedometer needle will bounce around when I hit a bump driving. When it does this, the needle always ends up at whichever speed I’m at. So, 1) what’s the problem; 2) is this dangerous and 3) what kind of repair costs am I looking at?