What material is the cap and rotor and how old is it? If it is a cheap cap and rotor (contact pieces made from aluminum instead of brass/copper) they will wear to a point and then just not want to run anymore and will show like things are normal (as the resistance of the plug in the cylinder is too much). Try replacing with a new cap and rotor made of good material from a good manufacturer, not one at pep boys and such. Plugs gapped correctly? As stupid as this question is but are the plug wires fully on the plug after you put them back in?