A Haynes comic would help you no end

Sounds like it's most likely the MAP sensor or its vacuum hose as it revs fine with no load on the engine. Most likely as soon as the load on the engine gets to a certain point, the reading it sends to the ECU is wrong, which is why there's a flatspot, as the ECU is only injecting suffiecient fuel and fiddling with the ignition to suit that incorrect reading.

Replacement's the only way to fix them.