Hello all!! Brand new to the forum, thanks for having me!!
Kind of a long post here, I apologize in advance. I have a 1999 Chevrolet 2dr 4wd Tahoe with a 5.7L Vortec. A couple months ago I was having some heating issues and so I started out with replacing the heater core. While doing so, I discovered the coolant system was filthy with what appears to be old dex cool and possibly rust chunks all throughout the system. So, I replaced the heater core, water pump, thermostat, radiator and also the coolant temperature gauge/switch. While I had everything apart, I took a hose and flushed everything as well as I could, and at least got clear water moving throughout. After buttoning everything up, the heat works well, however, the truck is running around 238-240 degrees. Thinking maybe it was a bad sensor, I replaced it again.......same result. I purchased a laser thermometer and noticed that the temp was indeed staying around 238-240 at the head near where the coolant switch screws into the head. The passenger side was fine at around 195 degrees.

So, next I discovered that the upper radiator hose was getting warm, however it never builds up much pressure at all. You can even pop the radiator cap at 230 degrees and no pressure at all. And you can stick your finger in the coolant and its luke warm at best. I also noticed that the lower radiator hose is almost cold. Well, I assumed maybe I got a bad water pump, so I took the pump back and got another one and replaced it. Still, no cure.

A few things to note, no water in the oil or oil in the coolant. Does not seem to be losing any coolant. Truck runs pretty well, no skipping or rough idle, etc etc.

Any advice on what to go after next?
Possibly head gasket and/or intake leak?
Is it possible the head or block water jackets are plugged up on the driver side?
Any and all advice would be greatly appreciated. It'* beginning to get the pride a little haha. I'm almost to the point of doing what every man hates to do, admit defeat and take to a shop. Lol!!

Hot upper hose and cool lower. Your radiator is removing a lot of heat. I think you are good.
Maybe a hot thermostat?

I have actually tried multiple thermostats from 180 degree to 195 degree and even took one and drilled some 1/8" holes in it to see that would help. Still says running 230-240 degrees and there'* barely any pressure on the upper hose. The upper hose never really gets as hot as it should and it stays soft even when reading so high of a temperature. What is strange is the external thermometer only reading 230-240 on the driver side head. The passenger head stays around the typical vortec temp of 190-195 degrees. It seems as though there is not enough pressure for proper flow and the water is not circulating around the driver side head as it should. Possibly a stopped up water jacket on the driver side??

I have read that exhaust gas getting in the coolant system could cause issues with pressure and therefore interrupting proper coolant flow. I just don't understand how that would happen without losing coolant.

Exhaust gas getting in the coolant won't interrupt flow. It just adds bubbles.

Speculating here: I hear poor maintenance of cooling system then a flush, so I think something is blocking a passage or multiple passages. For this I'd flush backwards and forwards a bunch of times, maybe even pressure flush it. If you pressure flush it, be careful not to apply too much pressure or too sudden of pressure, especially with how glorious the intake manifold gaskets are on these engines.

. . . and by glorious I mean weak.