[mutantenemy]

I have to agree that it is probably related to the fuel injectors and may stem from the use of the additives. I had a similar problem that developed after years of using fuel system cleaners and may have been due to using those products. I have also experienced that problem directly after using fuel system cleaner. When that happens I believe it is because the fuel injectors are becoming clean.

At any rate, that engine is known to have issues with the #3 and #5 fuel injectors failing regardless of additive use so you may want to focus your analysis on those two injectors. After replacing those two injectors (and all injector o-rings) my engine ran fine again after driving them in.

The injectors began failing around 100,000 miles, but I read on here that it has happened as early as 80,000 miles for some people.