T1 follows a long straight, right? If so, you've just burned off a large fuel load at a high rate, and you could be losing fuel pressure faster than you can replentish it. This affliction would not manifest itself in the infield or the paddock, as your duty cycle and hence your fuel consumption rate would be lower. The fuel pump, filter and supply line are likely candidates. You may also want to inspect the fuel tank vents while the tank is off to ensure that your fuel pump isn't drawing a vacuum in the fuel tank.
It's been a while since I've read an RC51 service manual, but I think the troubleshooting section specifies procedures for both pressure and volume tests for the fuel system.