Every half hour or so (20 - 50 miles of driving), the engine starts misfiring. Under the hood, I can see that the plug wire assembly (wire, boot, the connecting stick, and the puck at the end) has popped up 3 - 4 inches. I push it down again; it seems like the wire snaps into place on the plug, and it runs fine for a while before it pops off again.
Any idea as to why this keeps happening?