1999 Renault Megane faulty speedometer

We recently bought the above car only to find that the speedo keeps fluctuating between speeds, one minute its on zero then itll jump up to 80 and keep jumping in between zero and 80. we also noticed yesterday when we travelled about 140 mile round trip it only calcuated that we had done about 40 miles, obviously begging the question as to whether the low mileage it was advertised with is actually genuine. any ideas to what this problem is and whether it will be a big and expensive job
Thanks