Have you cleaned the carb and tank? I had a problem something like this with one of the newer Briggs with plastic carb. It would run fine, then act like it was out of fuel, sputtering a bit, then dying. I cleaned the carb. The bowl had water & lots of trash. I took the carb apart, poked wires through various places, and reassembled. Ran fine for a while, then did the same "running out of fuel" thing. I drained the carb bowl, and the gas looked good, but it was full of trash again. So I took off the tank, cleaned it and the fuel line. It's run fine since then.