Gas gauge not working

My gas gauge will not respond properly to the amount of fuel used.  It will register "FULL" until I have gone a good 40 miles or so, and then suddenly drops to about 7/8.  After another 50 miles or so, is suddenly drops to just less than 1/4 tank used.  Any suggestions?

When I got my car in oct 2016 the gas gauge was at half I filled up and the gauge went to full and never moved so I used the trip computer to let me know what was going on.  Two days ago as I was driving the gauge was at half which I verified by the trip computer.  I stopped for gas and it took 11 gallons.  I have always used an injector cleaner in all my cars with every fill up I also use now and then Marvel Mystery Oil in tank.  I think that these additives loosened the float so that it is working. I also use fuel stabilizer now and then. When I got the car it was averaging 12 miles per gallon today I get 20 miles per gallon it did clean out the injectors car runs great. Since yours is not stuck completely I would put some Sea Foam in your tank and some Marvel Mystery Oil!!!!!! You have nothing to loose at this point.  I will see what my gauge does in the next few days if the damn rain stops.  Take care.