Very common laurens!
Search through this same sub-forum using keyword "fuel" and you will see many hits!
Common issues...the sender itself is internally rotted,...the circuit board where the fuel gauge pod connects is oxidized and needs to have the contact points cleaned,...the contacts for the "cvr" are oxidized and need to be cleaned.
It's possible...that if the po had issues with this, the sender was removed and then the float arm "adjusted" (by bending) in an attempt to correct or calibrate the reading and in doing so, made it worse!
And another possibility...the float on the sender has become "heavy" and it's buoyancy is now wrong, it simply can't "float" at the correct position.