Electrical potential (the spark) will follow the path of least resistance. Things that will create an easier path at the coil are as follows: -- Bad plugs with way too wide of gap. -- Bad wires. -- Contamination on the surface of your coil like soot or metal dust). -- A breakdown in the coil insulation.
I'd check out your plugs and wires, clean the coil and if the problem continues then replace the coil.