Oh man glad to hear it was a radiator fix! I was thinkin' you had a whacked engine

 

Update - I haven't updated this in years but here's the rest of the story. So after I got the radiator replaced I had no problems for about a year. The engine was fine but then it started to overheat AGAIN. I would have to top the resevoir off with water every couple weeks to keep it running at normal temperature. I then decided to take it to the shop again and guess what they told me - I need a new radiator (they showed me how clogged it was so I know they weren't flat out lying)! So I had the radiator replaced again and after I left the shop, the car pushed all the coolant out after the drive home so I took it to the dealer next morning and they replaced the resevoir cap. After everything was running ok for about a month I traded it in for a Camaro SS last year. Was glad to get rid of that lemon and never buying a used car again. It was amazing that no one ever figured out the real problem.

 

The way you described it sounds like a bad head gasket as well. That will cause the fluid to blow out of the reservoir due to the pressure build up. Radiator clogging could be due to that as well - junk getting into the coolant, air getting into the system - I had that happen on my car (under warranty), never overheated on the gauge, but would blow all the coolant out of the reservoir - it also never showed up in the oil (i.e. oil never showed the milky condition evidence).

Similar thing happened to a friend as well. His reservoir would expand but not leak, just bulge. Had it up on a lift and spotted the leaking head gasket. No coolant evidence in his oil either and no indication of overheating on the gauge.

 

I have an 07 and the coolant is as fresh and new as the
day Ford put it in there. You sir have some major problems
if the coolant is going bad and the radiator is plugging up...
A CO test of the air in the radiator overflow will show a
leaky head gasket pronto. This should have been done
by the mechanic during the diagnostic portion of your visit.

Your Camero SS has more of a chance of being a LEMON
than your Mustang ever thought of being.
Bad mechanics dont make a vehicle a LEMON...