I had a similar problem at one time. Bought some ac Delco rapid-fire #5 spark plugs to trouble shoot. I read in the forums that the iridium plugs are good for 100,000 miles, so I looked every else for the problem. Then I found out the 5's are a direct cross reference and figured spending 25 bucks for a set was worth the troubleshooting. Plugs took care of the problem. Ran them for about a month while I ordered a new set of factory iridium. Now I keep them on hand so I have a known good set for trouble shooting.
