I woud just replace the plug and see if it repeats. Plugs do go bad do to cracks in the insulator, deposits shorting it out, etc. If the new plug fouls out in short order then you might have a bigger problem. This assumes that your points are set right, wires are good and the cylinder has good compression.....etc. Here is an excellent article about plugs, it covers everything:
http://www.aa1car.com/library/sprkplg2.htm
http://www.aa1car.com/library/sprkplg2.htm