my trouble is my 1999 ford expedition. after getting a needed oil change and tune up done.. (a stricly oil change place did the oil change, hubby did the tune up). the problem is that the wire to the number 7 spark plug will not stay on the plug.

pretty much every other day, I can feel the rumble and hear the ticking as the spark from the plug to the wire cap is arching. then I pull over, pop the hood and crawl into the engine to push the wire back onto the plug. I am sure the site of me hunkered on top of my engine must give some passerbyers a good chuckle, but its aggrevating me. any ideas as to why it is doing this?

hubby and I were discussing it and thinking that either the plug is not good (even though its new) or the wire itself is not good (even though its new as well). thanx for the help.