Don't know about that resistor but this story could be of some interest to owners of older MB's.
My W124 300TDT's fuel sender was also fluctuating below 1/2.
I am always adding an additif to the fuel when filling it up for cleaning the injectors and to prevent (to mutch) smoke.
A few weeks ago I was pleasantly surprised : during a ride of about 800 miles, I found out that the fuel gauge was again working as a new one and I didn't touch it at all !
BTW the additif is Marly CT1...
That was my easiest repair...