Have you had the plugs out or had the wires off. Sounds an awful lot like you have number 5 & 7 plug wires switched around. At idle or very light load it will seem to run fine with a virtually unnoticeable miss being that the two cylinders are next to each other in the firing order, so wires being crossed puts the firing very close but not quite where it should be resulting in a miss under load much like you describe.