If the problem is coming and going the easiest way to narrow it down to sender or gauge will be to measure the resistance of the sender at the top of the fuel pump assembly both when the fault is present and not present.
You can get access to it under the rear right hand seat by lifting the carpet then pulling out the big plastic bung. The outer two pins on the 4 pin plug with the thinner wires are the sender, so unplug the plug and measure the resistance:
The sender is just a variable potentiometer and it looks like it can be removed from the fuel pump assembly and replaced separately as it is just held in with small clips and has two soldered wires:
I have two spare complete fuel pump assemblies for the Series 2 Xantia V6 one of which is in the photo above. The resistance values are 350 ohms (or greater) when the tank is empty dropping to about 50 ohms for a full tank.
Interestingly when measuring the two units I have it seems one has a dodgy potentiometer - rocking the float arm slightly to the sides causes the reading to go intermittently open circuit - which would cause the fuel gauge to read empty as higher resistance is less fuel. This could very well be the problem you are having.
On the second unit I have the reading does not go open circuit when I rock the float arm to the sides so the first one either has lost the tension on the spring contacts or has a lot of play in the pivot for the arm. (or both!)
Should be easy enough to confirm from the resistance reading if it is the sender or not - given what I just saw with a dodgy potentiometer on one pump assembly my thought is faulty sender is more likely than faulty gauge for an intermittent empty reading!
