I put new spark plugs in last summer during my 120k service. Drove to Las Vegas for the convention and used a lot of octane boost on the way there and back. Also used a bit of race gas in LV. Soon after returning I had bad miss problems that ulimately were traced back to varnish on the new spark plugs. I think it was caused by too much octane boost in very hot weather.
Karl
S.I.E.B.E.R.T.: Synthetic Intelligent Entity Built for Efficient Repair and Troubleshooting