I wouldn't think a fuel additive could cause such a problem as it is a very low concentration added to a full tank of gas.
Your problem may just be a coincidence that its only happened since you added seafoam??
Have you checked your dissy cap and rotor??, remove and have a good look at it, look for white/grayish track marks and general arcing evidence, if it is still fitted you will have to remove outer casing from cap (black cover over orange cap), so you can check each lead post. check this first and post back .