Thanks for reporting back. As I was reading down through your description I was thinking it was combustion chamber carbon buildup. With that many miles on a motor, its usually the root cause.
What I usually do to all my cars: twice a year I run two cans of chevron w/techron FI cleaner in a single tank of shell 93 (v-power)
I've done that for years and have never had a problem. And the engines I've torn down all are very clean. Only once did I see a little coking on some valve stems. But it still wasn't enough to cause problems and was very minimal at high mileage.
btw, FI cleaners are not all the same. The "techron" cleaner is good stuff in my experiences of using and rebuilding my own engines.
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