Inspect the coil terminal on the distributor cap, a black ring begins to develop where the boot ends. This is carbon buildup, once it accumulates it can/will provide a shorter path to ground causing the electicity to jump to the black plastic insulator. Beyond that the next thought might be the ignition coil. Either of these can be easily tested with a spray bottle filled with water, just give both items a little spritz and watch for any adverse reaction.