Fuel Gauge Intermittent Accuracy

I now have 98,700 miles on a 2008 Prius, and have the 100,000 mile warranty. So, I'm coming down to the wire on getting a fix on my fuel gauge. Since I hit about 70,000 miles, my fuel gauge will sometimes drop to the flashing bar reading, appearing that I'm about to run out of fuel. Without filling back up, upon restarting the car, it will then go back to a normal reading based upon the amount of fuel left. I didn't want to hassle with a dealer visit over just that issue, and knowing that Murphy's Law would apply and therefore the problem would dissapear as the dealership came into sight. Eventually the problem coincided with a dealer visit, and they didn't find any problem, said they added a charge to the auxiliary battery and it seemed to fix it. At around 80,000 miles it started up again. At 95,000 miles, and before a long road trip, I had it to the dealer again, and decided that replacing the auxiliary battery wouldn't be a bad idea (I live in Arizona, so nearly 5 years from a battery seemed reasonable). They replaced the battery and "performed a reset process" on the fuel gauge. That worked for about 10 days, and now it is back, intermittently. I had a scheduled appointment today, and the problem disappeared again. I am hoping to get to the root of the issue before the warranty expires. Any help out there?