I forgot to mention that I had the expansion tank replaced for just the situation you described, Mike. On searching some threads, I did see a few posts regarding "air" in the system. Things were fine up until the week after I had the system flushed. Can you get air in the system that way, and how do you evacuate the cooling system if air is detected?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car