Out of control Fuel gauge

Evening all

My fuel gauge has decided to become wiled and out of control.... the tank is half full and will read half full then all of a sudden it will wonder up and down the dial then go back at its original position. Any ideas

How old a vehicle we talking about? Older cars used to have a voltage regulator fitted to control the gauges - just a thought.........

it is a 1986.

Slight modification though i should have said. I have put a new chassis on it and fitted a td5 chassis with a rear fuel tank with a 300tdi disco sender unit in it.

My TD5 gaude is just as cranky.
Given up trying to solve it.
Also my fuel light comes on when 3/4 full and goes off again at about 1/4 full ??????

I bought a new sender for mine (as sig.) when it started doing this. After taking the old sender out it was apparent that the wire connection to the float level wiper bit was frayed and had about 1 strand connecting it .. simple solder job cured it ..

Mind, it's a beeatch of a job to get it out in the first place ...

same prob on my disco found out the live feed from bat to igniction had coroded well all 5 had and it would not start and windows didnt work

Same problem, changed the sender, 5 min job, £10 part, job's a goodun