Pontiac Repair: Pontica Grand Am Problems, pontiac grand am, coolant

Question
Hi there,
I have a 93 Pontiac Grand Am SE and to tell you the truth know nothing about cars!
About a month ago I took it in to get a coolant flush because the mechanic said that the coolant I had in it was old and not lookin to good! Following the coolant flush I drove it home and parked it and left it for about two weeks (I went on a vacation). When I got home and drove it again it started overheating. Prior to the coolant flush the car had not overheated.
It takes about an hour (maybe a little longer) to overheat and actually have the coolant light come on but all the while the gauge is climbing slowly.
I have had the thermostat changed but the same thing continues to happen.
When I got the t-stat changed the mechanic told me that the rad is still really dirty (this is after I have had it flushed)and he recommends getting it done again.
I am just curious if a dirty radiator could be the culprit or if it is something more. I'm just wondering if you have any suggestions on what I should do.
Thanks,
Amy  

Answer
Hi, before you have it flushed again, need to check for leaks, you could be loosing coolant and this will cause it to over heat, make sure that the coolant fan is coming on, if the mechanic used a cleaner in flushing the radiator, what this could have done, is loosing any build up in the radiator, and causing it to clogged the radiator, and this would cause a over heating. If this is what is wrong, then you would have to have a new radiator.