I would have to go along with the thinking of Beast here. when you load the engine it is easier for a wire, coil, plug wire to jump to ground. These are hard to find sometimes, because your circuits will test ok with no load. Try using a fine water mist and put a load on engine in garage and turn off the lights. A lot of the time you will get a light show to point you in the right spot of trouble. The misting should keep you from burning up the clutch. It helps the jump to ground with less load. It's like a coil breaking down, they run at an idle but load them and watch them fail.
I had this after an engine change once and it was a slightly pinched wire off the distributor. Couldn't see it or test it till the water and light trick. Must have done it taking it out or putting it in.