Last year my parents bought me a 1977 Bus for my birthday/graduation. My father drove it the 130 miles home without a problem. A couple of weeks later though, I noticed a slight hesitation here and there. It quickly became worse. It would surge even though my foot was steady on the gas pedal. Then it would eventually stall and die. When it cooled down again, the Bus would run fine until it got warmed up. A friend of mine and I spent some time with it. We replaced the plugs, wires, condenser,, distributor cap, fuel lines, cleaned the injectors, and found that it passed all of the amp, volt, and compression tests we could throw at it. We did notice, however, that it drops significantly in fuel pressure when it warms up and that when the pedal is a a certain angle, the Bus will literally shudder down the road. I've brought it to the local Volkswagen dealership and they replaced the fuel pump twice, cleaned the tank, replaced the airbox and head sensor, and have done an overall tune-up but nothing has helped. It will idle all day without a problem, but as soon as it is put under a load and it is warmed up, it hesitates and dies. Whats wrong with it?