The old fuel is very bad for your engine. Not only have most of the things in it evaporated leaving almost only oil, but it could be contaminated with dust particles and dirt. If you left it on a hard cold surface such as concrete, it also most likely caused condensation on the top of the bottle, which most likely caused some of the fuel to chemically react, leaving un-safe liquids to enter your motor. Get yourself a new gallon of fuel. What percentage of Nitro are you running? I would recommend either Sidewinder fuel, or O'Donnels. The reason your truck starts running differently at the end of the gas tank, is because since there is not alot of fuel going to the engine, it becomes "lean" for a few seconds. Lean is when your engine isn't receiving enough fuel. This causes the motor to struggle to stay on, making it run erratic. It is regular, and happens with all cars at the end of a tank.
