I just recently replaced parts of my cluster for this.
My new, actually used, cluster seemed to fix the problem. I think, for me at least, there was some short in the panel or even the gauge itself that this would happen to me.
My fuel gauge, when full, would go all the way to F. Then sometimes it would drop almost to E, then go back up like to 1/4 tank. Very intermittent readings.
I suggest finding a cluster, pluging it in and diagnosing it that way. If the new/used cluster gives solid reading...then you knwo its not the fuel tank sensor, to which I think is kinda hard and difficult to repair or replace.