@jso2378 : you could also feel the two hoses going into and out of the firewall to see if maybe one is hot while the other is cold. If that's the case, then your heater core is blocked up (with sediment). I don't know if this is a thing with Impalas (it's never been with my 2002), but it's another possibility. My '97 Taurus had this problem (chronically) ... so much so that I made a couple of remnant hoses with couplings so I could quickly hook up a hose to the spigot and blow the darn thing out! Used to do this twice a year IIRC.
But my money's on either the air bubbles or the Blend Door failing. Also had that happen with my Taurus. There was a cigarette pack-sized motor under the dash that moved the door when the Temperature Knob was turned. It was full of plastic gears and one of them had enough teeth crack off that it wouldn't engage to move the door! It was a $40 part, and a PITA to replace (upside down, draped body on floorboard and seat). In the Taurus, the Blend Door "defaulted" to the COLD position, so what I did for the first season was just prop it up under the dash with a small piece of plastic I cut off a detergent bottle ... jamming it in a slot to manually keep the Door in the HOT position.