Sometimes when you are at about 1/2 tank, you can experience 1/2 tank lean. This means that the fuel to air mixture gets leaner than usual, and could be setting off your settings. Try to richen the mixture a bit and see how it goes. Make sure to try and check your temperature with either a temp gun, or using the spit test. If the temp is over 260 or so, it could be that the mixture is too lean.