[Autoban]

I used to work for VDO and was the Supervisor in charge of the production for these sensors in the late '70s (so I made probable a lot of the sensors we are using these days). Knowing how they are made my first guess is that the wire on the internal resistor is starting to wear through and has some shorts that make the gauge jump around. It is a variable resistor and I don't remember the resistor value specification in 1 Bar increments for these senders, but if you can find the values you can test your gauge.
Buy cheap fixed resistors of the correct values and connect them (with clips) between the wire of the sender unit in the engine compartment and ground. Start with the 0 bar resistor and then check your gauge (with ignition on) to see if it reads close to 0 bar. Repeat the steps in 1 bar increments all the way to 5 bar. If your gauge is way off or jumps around, it's the gauge. If your gauge reads fine, the sender has to be replaced.

Juergen