'97 Mitsubishi Galant overheating. Plz help

Okay, awhile back my head gasket went bad, right after the cold weather hit. I had that replaced about 3 or 4 wks ago and all has been good.
Then 3 days ago it over heated, QUICKLY like after a few miles it went from normal to as high as it would go. I let it cool down and there was NO OIL, I put oil in it and filled up the radiator. Hoped it would fix it. I drove to town and the same thing happend. The only steam of anykind is coming from the coolant overflow. We went to replace the thermostat and there wasn't even one in it. The thermostat we got was for a 97 galant, which is what I have but it wouldn't fit. Were going to try to do it again today.

There is a ROUND THING on the far left if you facing the engine w/ SAE PA66 on it, and it seems to be leaking. I can't seem to find out what this is if anyone has an Idea I would appreciate it. There was no heat from the heater the first day it did this, but after it overheated twice from town and back there was heat coming out. Since there hasn't even been a thermostat in it, it shouldn't be the main problem should it? Someone said make sure that the water pump is still working, how would I check that?
Any other ideas would be greatly appreciated.
Thanx

EDIT

Also, when I drove to town and it overheated, I filled it back up w/ fluid, let it cool down and it was fine for about 3 mins, but while at a red light it went from normal to as high as it can go almost instantly. The coolant overflow bubbles