Hello,
The mechanic you spoke to may have been right, but his terminology was a bit off. By "sender" he must have meant the fuel screen located in the tank. Here is a picture:
This sits at the bottom of the fuel tank, behind a rubber cover just above the right rear axle. If the bottom part were clogged, I can imagine you might start to have problems at half a tank of fuel... when the tank is fuller, more of the screen would be submerged, so the pump could draw fuel from the unclogged upper section.
It should be easy to remove and replace the screen, there are several threads on the board that describe the process; it is also referred to as a tank strainer. You should also try to figure out WHY the screen got clogged. You might need to get your fuel tank steam cleaned if there's a lot of crud in it, or run some Biobor through the system if you have a fungus infection.
Good luck!
Nate