2005 Cavalier- gauges sometimes work, sometimes don't- frequently not right

If they zero out when i turn it off, and they happen to come right on when I power the car on, they are correct. if they happen to power on while i'm driving- they start from wherever they are- so if i'm going 30 at the time- it will read 30 when i am going approx 60... they come on and off- any ideas? just want to have an idea before i take it in and end up getting ripped off at the shop.
thanks!