[Kidd-7]

I have an 01 Impala with the 3400 engine; 74K miles. I had a heat problem about a month ago. The temp gauge would rise to about mid way very fast and the heat would blow cool, then the gauge would drop back down to normal and heat would be nice n warm.
I eventually did a flush n fill on the coolant, it was nasty brown, it took 5 gallons to clean out the overflow. This seemed to correct the problem, temp gauge has been stable, heat is hot.
Well yesterday I noticed the gauge being erratic, not to the extreme it was, but still moving more than normal. So when I got home I opened up the overflow and its bubbling, not boiling, bubbling like air is coming out of the system.
Any ideas, my friend who is a mechanic said the head gasket in these engines are common to fail, but when I flushed n filled I also changed the oil, its still clean.
Is it a head gasket letting combustion gasses in the cooland but not letting coolant in the oil?
It it possible I did not purge all the air out?
-thanks.