Well, my issue is different than both of yours!
My guage is ALWAYS on full! LOL. It just lays buried past the "F" mark. The only exception to that is if I really top off the tank. Then, the guage will go all the way to empty...then bounce back to full. That occurs for the first 50 miles of a topped off tank...then it goes back to rest at full! LOL. Most irritating part is the check guages chime everytime it sweeps back and forth for the first 50 miles of a full tank!
I am still on the original pump and sender. I've got 113,000 on the car and it'* been acting like this for the past 15,000 or so.