2004 Impala Getting Air in the Cooling System

Is there anything known about a 2004 Impala's (3.4 I think with 70000 miles) cooling system getting air in it? My gage starts rising when I stop and then goes back down upon driving. I had the radiator flushed and it worked good for about a month but I kept smelling coolant outside the car around the doors. A few days ago, it went back up to 215 degrees. I took it in to a different place and the mechanic said there was air in the system and whoever flushed it didn't bleed the air out. I don't think this is right because it was doing this before it was flushed. I think air is getting into the cooling system somehow. Right now it is fine, but I still smell coolant and there is no visible coolant leaking. Where could the smell be coming from and how is air getting in the cooling system?

Someone told me it could be the heater core. The floor isn't wet and I'm not losing coolant.

I took it back in to the first mechanic on Saturday and he said that they did bleed the air out, but there was a little air in it. This was 2 days after the other mechanic bled it out.

When the radiator was flushed, there was gunk in it. I have heard some horror stories about the Dex-cool damaging gaskets but I was thinking that GM corrected this problem on the newer model cars. This mechanic replaced the coolant with a uiversal brand.

Is the head gaskets causing this problem? I haven't noticed any steam coming from my tail pipe.

Any help would be appreciated greatly!