Cylinder 8 misfire - and it's self correction!!?

[Vamphry2000]

I own a 2014 Ram 2500 6.4L Hemi took it to the dealership for a heater core issue and was told I also had a misfire code on cylinder 8. Had them replace all coils, spark plugs, wires, and fuel injectors. After $7K worth of work I was told it was fixed. Went to drive it home and did not get off the lot and check engine light came on. Was told it was still cylinder 8 having issues. Did vacuum test which I was told passed with air but failed with fluid. Continued to diagnose and was told they could not come up with a definitive cause said they wanted to replace or tear apart my engine to diagnose. Mind you did not have a tick or rough idle before the dealership got ahold to it. I told them to put it back together I had lost confidence in their ability to correct. Went to drive it home and the truck ran like crap very much a misfire present the check engine light started flashing which it never did very rough idle. Took it to a separate auto shop they seen the code noted the rough idle did a diagnostic and gave me a bunch of reports told me it was a valve train issue I could either replace the engine or spend several thousand dollars tracking down the issue with the misfire. Again told them to pack it up I was just going to trade the truck in for a new one. Drove it off the lot no misfire but the yellow check engine light was on. Drive it several other times no misfire ran beautifully. A couple days later the check engine light has shut off and is running like before. How could this be a engine replacement or serious valve train issue? Those don't fix themselves. Please help figure this out.