Be sure your engine is grounded well.Check all the grounds.
It could be a bad Ignition coil or module, firing when it isnt supposed to. An open winding in a coil primary can fire at the wrong time due to vibration across the broken wire. Also a module, which controls the coil primary, can have similar issues and cause a crossfire or incorrect firing time as it can open the circuit to the coil primary at the wrong time causing a ealry/late/crossfire.
Also a cracked dizzy cap can cause a crossfire/misfire.
Look around for carbon tracking. look at the cap and rotor.
Maybe replace the coil, if that doesnt do it, try a module.
Fact is stranger than Fiction