p0303 code

I got this code and is says misfire on the 3rd cylinder, check engine light is on, and the jeep seems to run rough and hotter. went to the dealer, they said probably no big deal see if it clears . Here is the thing though I have 165,000 on the motor it has been well maintained, and always checked by the dealer. As you all know you know you vehicle, you know when there is something wrong I think there is something more major wrong, I had the jeep checked for excessive oil consumption and they just checked the level at after a 1000 miles said I had an oil leak fixed that and not to worry that was 2 weeks ago. MY gut feeling is the engine is ready to give up the ghost they the dealer knows it and am hoping I will buy before it does. ( maybe I am paranoid, but ever since my go to guy service manager left, I have felt like the rest don't give a hoot) I am kinda hoping the engine does go because then can get them to replace and sell it privately for more money than I would get if I traded it in. because if I trade it in I will get really dinged on the mileage but with a new Chrysler motor well that is a Jeep of a different color.
so here are y questions. Those of you with high mileage over 100k who thrown this code what did it turn out to be?
and those who have thrown this code at mileage what was it?
how likely is it that rings of valves are going to be bad?

Specifics Jeep JKU 4 door rubicon 165k on the motor about 90 on the trans which is also starting to make a funny noise. The dealer told me it was my suspension.. BS I know a jerk in the tranny from a suspension shift.

what should I do?

Have you changed out the spark plugs and wiring yet?

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Does the engine run smooth at higher RPMs? If so, it could be a broken valve spring. Could also be dirty injectors. I'd start with removing the spark plugs to check for proper wear.