Hi and welcome to the forum!
It sounds like you have an erratic fuel level sensor, or the wiring leading up to it. As you say it happens intermittently it will be a difficult thing to prove to the dealers.
The only option I can think of would be to mount a little camera on your dash and have it record the whole time whilst pointing to your fuel gauge in the hopes that it exhibits the same issue.
Otherwise (and this is a long shot), maybe see if you can borrow someone's OBDII reader and install Torque on a Android phone or tablet and set it up to log the fuel level data. Hopefully this will be able to prove to the dealership that the sensor is faulty and giving false readings, but I am not sure whether they would accept log files as proof - my money would be on the camera option; at least then you will have photographic evidence of the fault that they cannot seem to reproduce.
Good luck and let us know how it turns out
