Going off of what was stated with contribution failures from different cyls. An intermittant electrical issue, ie uvch, would do just that. O-rings wouldn't be intermittant imo.
Had very similar issue. Blowby increased exponentially. No codes and got it to fail #2 on a cyl contribution test.
Tested engine harness to uvch's for resistance readings. Everything read 3.2-3.5 except for #4. It read 8.5 I believe. Changed out driver's uvch. All injectors tested 3.0. With new uvch got 3.0 from all 4. Retested. Thought all was good cold. Worse readings were only 3.2.
Started it and was letting it warm up and it would intermittantly miss while holding it at about 1800rpm.
Shut it down and immediately retested my harness. Driver's side was still at 3.0, all 4 BUT the passenger side read 6.5 each. Grabbed my son cause I thought I was losing it and low and behold the passenger side now read 3.2 each. Installed new uvch on that side, all inj's tested 3.0.
With new uvch's, both sides read 3.0, the same as testing at the inj's.
At 30* the truck ran better starting than being plugged in or oa temp at 75*. Smoked less cold than it did warmed up.Cured my blowby issue that looked like a fog machine.
It is really simple to pull the engine harness connector and test with an ohm meter but use a really good meter that has a 0-200 scale and test your leads for a base reading.