If the 2.5 in question is in a T Maxx, the good news is the problem is the fuel tank. The bad news is, it's not a fixable problem as long as you run the stock tank and stock 2.5 carb. The combination of tank shape and pressure demands from the stock carb will never allow it to work right once the fuel level falls below the narrow top portion of the tank. You can run a different pipe that delivers more tank pressure, but your 2.5 will still run hot when the fuel level falls even if it does idle. The two most effective fixes are either an OS carb or a shorter, wider fuel tank. The carb fix is simplest and is nothing more than a quick swap and retuning, but it's also expensive unless you can find a low cost used OS carb. The tank fix is much cheaper, but involves moving things around on your chassis to make room for the new tank.