I would also advise checking to see if your Aux. fan is kicking in at high temps to prevent over-heating. If no, this could be a variety of things, as well...
- "212" (100C) switch
- Aux. fan relay
- AC Pressure Switch
- AC Pre-Resistors
Search the posts here...I found a ton of info already posted about coolant problems before my own flush/sensor replacement this spring.