Fuel gauge problem on peugeot partner 600lx van??

Hi

Recently brought an X reg Peugeot Partner van but the fuel gauge has a mind of its own!! When you first start the van the gauge flicks up and down really fast, empty then fuel empty then full. This happens so quick the needle bends when it hits the stopper!!

This carries on for about 25 miles then it sorts it self out. Anyone got any ideas? Im not even sure if the eventual reading is accurate or if it just stops at that same point all the time.

Its a bit of an inconvienance not knowing how much diesel you have!