Noob, try this, check the tank lid or the pipe between the pipe and the tank, also try to check that you dont have a blockage in the pipe nipple going to the fuel tank could be that, maybe the motor is struggle to get fuel and is going lean or is that not it? I am not 100% sure, but will help to check it, also a little tip, if you have a pipe that is two parts, put in a longer piece of silicon tube between the header and the pipe, this will give bottom-end, and shortening will give top-end, but TEST before you race, you dont want to over do it and find out on the track. this may your ticket to getting your motor to peak where you want it to. I am a bit confused with the symtoms you describes, my advice, recheck the WHOLE fuel system, have you got a fuel filter, if so, clean it, if not, check for tiny particles that could have caught in the carb inlet, or in the bottom of the tank (more likely as it runs ok for a bit) then theck from the pipe to the tank, all connected, no blockages, to leaks, then check the tank lid, make sure it seals properly. Please let me know. This is interesting...