That gets my vote. Those tanks often start suckin' air at that level. Associated has had issues with the RC8/RC8T tanks as well as RC10GT2 tanks. When you're running your car and it starts to lean out when the fuel gets to that level, imediately pull over and look at the fuel line. Does it have a lot of bubbles?

One other thing...When you tune your engine are you doing it with a warm engine and a full tank, or with a hot (5 laps or so) engine and 1/2 tank? Tuning at 1/2 tank will lessen the lean out. This is assuming that you're not having tank problems...