I would suggest to you that there was and is something wrong with the car, or the gas station your getting that 87 octane from. It of course is a given there is no modifications done by either you or any prior owner or dealership, the engine is the 340-hp 5.7-liter V8, and that the car is at least a 2005 or newer. I see some problems reported with engine/fuel problems here ( 2005 Chrysler 300C Problems, Defects & Complaints but the quick research I just did is in no way a definitive result). By law, gas is randomly tested at various points throughout its distribution system for quality and water/contamination within. If your putting 87 in from many different stations geographically separated from each other over a period of time covering multiple sources, and your not 'beating' it all the time with WOT and highly aggressive accelerations, and from the beginning you have taken care of the basic oil/filter maintenance requirements (good mechanical condition), then it (in my opinion) is something out of kilter in the car your putting it in. Since it is that old of a car, it is way beyond any hope of having it fixed under any warranty unless they accept responsibility (unless it is UNDER some kind of documented recall or emissions warranty and they can duplicate your results which is HIGHLY unlikely). I doubt this can be fixed without you incurring a large fee.