Ok, I have an 03 coupe with 51K on it and had the radiator replaced about 20K ago because it was cracked from debri on the road. Heres the scenario:

About a couple weeks ago I noticed my car was starting to run warmer, wasn't to concerned as it has been warm and I've been running my AC alot. Well its gotten progressively worse.

While driving, the temp gauge will start rapidly rising then slow around the top line before the H Mark it then a couple seconds later, it will drop down to normal. And a few minutes after that it will repeat its self. It doesn't matter if the AC is on or off whether its hot or cold outside it still does this. Today it was sticking on the higher side for a long time, so I turned on the heater upto 90 and turned off the AC button, and COLD air was blowing out of the vents?!?!? I checked the fluid levels when the car is cold and they're perfect. I'm hoping that its just the thermostat, but any one have any ideas of something else it maybe? I was just completely puzzled today when the heater was blowing ice cold. Anyone know if its covered under the 6 year 70K Engine Drivetrain warranty?

Thanks for the help...

 

You've got air in your cooling system. You need to fill it with coolant (or distilled/deionized water). Your heater is blowing cold because there is no hot coolant getting to the heater core (e.g. air bubble)

 

There's a machine that allows you to bleed the air out of the coolant system, I believe most shops have it. Try having that procedure done to it. I remember when I did my first drain/ refill, the car started to overheat right away, so I took it in and they bled the coolant for all the air bubbles that had been trapped in...