I just replaced my 105's fuel gauge and fuel tank sending unit, because of similar symptoms as yours. The opening for the fuel tank's sending unit is directly under the winshield frame.  It is a bear to get in there.

I would start in the tank first.  You can buy a fuel tank sending unit cheaper at a boat/marine store than special order from the White guys. It's the same unit at less than half the cost.  Keep your old gaskets and bolts. If the gauge moves its probably ok.

 It's a lot of work, or Keep a yard stick somewhere outside the cab to measure once in a while.

Good luck.

Larry