140k miles, G37S Coupe, been misfiring for a little, trying to figure out what it is before I go blowing money. Im assuming it would be my spark plugs, coils, or injectors. However, today I filled it a little with gas (93), drove out and the car had absolutely no misfires and ran perfectly. This is the first time I have ever had that happen to me in this car. I drove it to work where it sat about 6 hours off and when I got back in hoping it stayed the way it was, the misfire was back, but it felt not as severe as it did before. Any one had similar situation like this? Again it's most likely one of those 3 things which are all going to eventually get changed out soon but just would like to see if any ideas from you guys. Also have a rattling from under my car at times, sometimes will randomly go away, most likely something loose like my heat shields which is a known problem with this car. However, as the misfire went away so did that rattling, could just be a coincidence, or not as I said sometimes it randomly goes away. I have had the car about a month, I know for a fact the guy before me did absolutely nothing to take care of the car. Already replaced a fuel tank, diff bushings, and driveshaft. Sparks, coils, and injectors I doubt have been touched. Not even sure if he filled it with the right gas which could also mean that the car is finally flushing out all the 87 he filled it with, if he did, and finally using the 93? As I said all of the above needs changing anyways, just want some other opinions on it, really weird situation.