I have a 2000 concorde that started overheating a few months ago. Here is the work that I've had done so far:

replaced t-stat twice
new water pump
new drive belts
new timing belt
new coolant overflow/reservoir cap
had coolant system flushed
have bled system numerous times for air pockets

When problem first started, the car would only overheat if I was sitting still. Now, it overheats after 10 - 15 minutes of driving. Mechanic at chrysler dealership was unable to find any leaks during diagnostics, could only determine that t-stat was bad so I had it replaced again. Yesterday my heater stopped working & only blows out cold air. I also heard what sounds like water or fluid moving around behind heater vent. The coolant in overflow tank boils and skeets out of overflow when car gets too hot. As far as I know, the fans are working as they should. I haven't noticed white smoke or milky oil, but I know it could be a cracked gasket.
I've had the car for 3 years & probably need to change radiator hoses. I have spent a load of money trying to fix this problem & I don't know what to do next. Any suggestions? Could a bad heater core cause the car to run hot? Anyone else had similar problems? If so, what could be the cause?

Thanks

 

more then likely warped heads

 

I know this is not what you want to hear but I had the same problem on another make(Volkwagon)overheating and blowing cold air and did the same as you,change the thermastat twice,had it flushed and brought it to the dealer and they couldn't find it so I brought it to a radiator place and had it checked by them and found it right away....it was the heater core.Have it checked by a professional rad place,thats all they do.

ps-I lived in montreal at the time and the place was called"Mr radiator"

 

I really appreciate you guys taking the time to respond. Your suggestions were very helpful. I'm hoping that it's not the heads, but I'll have the heater core & hoses replaced first to see if that will fix the problem. I'll let you know how it goes.
Thanks again!