I did a compression test on my stang last week. I then re-installed the original plugs since they were all burning clean. However, now the #4 plug (passenger side) wire keeps popping out at the head. I tightened the wire's socket with pliers but it still pops out when I drive for a few miles. I was very careful when installing the plug since I know the threads strip easily. The plug itself appears to be straight and snug. What do you guys think? Thanks for your replies.