Risa,
If you only changed the spark plug, the problem will come back. It's a matter of carbon tracking on the spark plug and/or spark plug wire.
Try this; replace the spark plugs and wires. If you only replace one, the carbon track etches itself onto the other component.
I have seen this many times.
Look at the spark plugs when you take them out. If there is a black line that looks sort of like a lightning bolt, from the top of the plug, down the white ceramic element, then that is a carbon track. The plug and wire must be replaced. If you only replace one, the carbon will transfer to the other. This will become an endless loop.
Replace the wires and the plugs.
GMdude