It's possible that the radiator cap is malfunctioning but I replaced the original cap with the one from my 02 Dakota which about 100k less miles than this truck.
In reference to the HG, I have run the truck cold with the cap off in the past. The coolant isn't very turbulent, but there are intermittent bubbles that keep rising up. No matter how long I run the truck, the bubbles keep slowly coming.

 

Unplugged the O2 sensor above the cat on the side of the misfire, then the one behind the cat. No change. But I did notice something that sounded like a slight exhuast leak from that side. Upon closer inspection, the noise is coming from what seems like the valve cover, near cylinder 5.

 

Wonder if you have a collapsed lifter..... Compression test?

 

Oil filter? It's nowhere near cylinder 5. I was unable to compression test due to the spark plug adapter piece being too wide. I could get it in the hole, but absolutely no way to slip a socket over it to screw it in

 

Not filter, lifter, as in, part of the valve train. Yours is overhead cam though, so, the lifters will actually be under the 'valve covers'.

Get a different adapter. My gauge has a sparkplug-looking end, a short length of rubber hose, and then a quick connector. I just screw the hose in by hand, hook up the gauge, and go.