2005 Chevrolet Impala overheat at idle

The problem just started recently. At idle the temp gauge will climb into the red. Once you start moving or the engine rpm's climb the needle on the temp gauge goes down. At the same time there is no hot air from the defroster or heater. The hot air does not show up after a long while, even if the temp gauge is in the red. I have replaced the water pump & t-stat. I even flushed out all the lines and radiator thinking they might be block. The lines and that all showed a good flow through. I went and bled the system as well to eliminate that possibility. I am stumped at this point, especially since one day the cooling system pulled a vacuum in the lines. I had discovered this when I was trying to remove the radiator cap and heard the whoosh of air and the coolant in the reservoir started to flow back into the radiator.