Other Fuel System Tank Assembly problems of the 1997 Buick Riviera

Consumer keeps a close eye on gas gauge. When consumer got out it was on a 1/4 tank, when she got back in and started it up, it jumped to 3/4 tank. Several days later, gas gaufe was on 1/4 tank and it jumped to full. Consumer took vehicle to GMC dealer, and service man showed a memo that indicated same problem because it was not a recall they were not entitled to fix it at their cost. Problem was not fixed by dealer, it still exists. When traveling and guage reached 1/4 tank instead of continuing toward empty it moves back toward full.