Overheating/Coolant flow problem

[700 sportsman]

Hi guys, I have a 1990 first gen 12v motor in my 1996 GMC. It was running good after the swap for about a month and then it had 2 ocassions where it overheated to well over 200 and pressure forced coolant into the overflow tank. I was immediately thinking headgasket, but then after a cold and hot pressure test I found the leak down to be less than 2 psi in about 25 mins. So I concluded my issue was else where. Then I flushed the entire system and replaced the thermostat, and all was good for a week or so. Ever since then it seems to sporadically overheat, which I can't figure out. I do have a second gen. thermostat housing to change the outlet angle, but I don't think that is related. Last night it ran fine, even slightly cool in about 80 degree weather and then after driving about 30 miles a short shut down and then in the process of driving about 15 miles home it got very hot and when I pulled into the garage the overflow bottle was almost full and I could hear steam inside of the head. Also when the bottle fills up it has some blackish residue in it, doesn't appear to be oil but perhaps exhaust? So I'm very confused as to what caused it from running fine to overheating?

My current thoughts:
- Need to add second gen. thermostat bypass hose
- Headgasket is leaking
- Serious air in the system
- Coolant flow issues (We originally had to raise up the radiator to get the level above the top radiator hose)

Any ideas or input is greatly appreciated