if your brother also removed his governor and has the same issue, then i think you need to install a return spring as is recommended and shown by buggymaster in this thread ...
http://www.buggymasters.com/forum/showthread.php?t=292
It might be running fine on the flats because the engine is getting a smooth rpm pull up to the top end vs really getting put to the test under a more resistive full load. (bumps/hills.)
otherwise, another item to check--is the fuel tank -- the rear push-in mount on mine was all chewed up and wouldn't stay in the engine cover slot. the tank apparently bounced like crazy and as soon as i ran about 1/4 tank of fuel out of a full tank, it would cut out on the rough stuff. Ran fine on the smooth. I bungied the tank down and problem solved.