"Future" fuel gage problem?

[86560SEL]

I have never noticed it before, but when I started my car this morning I noticed that a couple of times that the needle on the fuel gage went all the way past the "R", as if the ignition was off. It did this twice briefly, then it went back and has worked fine since (at least today). What is the possibility here? Gage going bad, unit in tank bad, or loose connection/ground? My old Pontiac does this from time to time, but it goes past "F". I have been told it was a loose ground, but I have not had time to check it. I usually keep at least 1/2 tank in it.

Also- even when my cars tank is completely full, it only goes a little past 3/4. Strange.

Thanks in advance