I always install spark plugs with a torque wrench, and during about a 4 month time frame on my 951 I went through 10-12 spark plugs because this same thing kept happening. After dumping ~$15 on spark plugs at one store I finally went to a different one and the issue disappeared. In my case I think they had been handling them too roughly before selling them. Not sure how much that applies to you, but there's my story on the same problem
