1997 Ford Thunderbird spark plug keeps popping off

Noises problem
1997 Ford Thunderbird V8 Two Wheel Drive Automatic

my spark plug number 5 wire keeps popping off and it seems like the spark plug is vibrateing inside the hole it's screwed into, and creating a popping sound and everytime I drive the engine runs rough and has trouble idleing. Tried to tighten the spark plug with a ratched it seems tight yet it still vibrates and pops the cable off.