The sensor it the tank probably needs replacing or adjusting. It has a small arm that moves on a conductive track to vary the resistance to give a reading and that arm can either wear a groove in the track or move away from the track so that it makes intermittent contact.
If minor bumps in the road make the gauge change this is a good sign of the problem. It can sometimes be fixed by bending the arm so that it makes better contact or if it has worn a groove bending it so that it is no longer in the groove. However, given the hassle involved in removing the sender to repair it, it best just to replace it. And while a sparking shorting sender unit arm sitting in fuel fumes as it makes intermittent contact with the conductive track is not as actually as dangerous as it sounds, it something I prefer to avoid.
Also check the wiring harness plug closest to the sender is not damaged or loose.
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regards Blue
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