I have the same problem with my 86 F250, there are multiple areas to look at. It could be the gauge cluster, the sender, or wiring. In your case it would most likely be the in tank senders. Have you checked out the Ford owners message board, there are many good writeups on the subject.
I don't know but it is strange. I have an '85 Full Size Bronco 5.8l and after about 6 years my gauge started acting up. In my case it goes from Full to Empty in 1/2 a tank. Then just before it is really "empty" it jumps back up to about 1/8th of a tank and drops slowly down.
90% of the problems relate to the in tank sender, most people change the sender when they do the tank, it starts leaking a few years after the gauge starts going bad(those of us in the northern climates). It could be wiring, it could be the gauge cluster but more times than not it is the tank. I have problems with mine, but I just switch tanks when the thing starts sputtering. Shortly thereafter I gas up.
I have an 85 f150 with the dual tanks, and you guys are lucky if your sat least tell you something, I'm in the dark.. The main tank gauge does not work at all... Put a new tank in and that s when it stopped working, Cause it may have somthing to do with the fact that I brok the electrical connector ..LOL....
I occasionally get a short and blow the fuse for the tank switching valve. I just keep a bag of 10A fuses on me. When the fuse blows it just defaults to the front tank, the rear is the one that leaks above half so it doesnt matter to me. When plowing I just check to make sure I can still hear the solenoid switching, every hour. If not I replace the blown fuse, if I have to use the rear tank.