Boxster overheating during DE Track Day

Overheating problem: Here's the situation - I participated in a DE track day a few weeks ago and experienced an overheating problem. The temperature gauge was all the way to the right and the red warning light came on. Once I pulled over the heat returned to normal. The ambient temperature was around 60 degrees (a nice fall MI weekend). Background: I replaced the water pump and belt this past winter. I followed the shop manual procedure for refilling and bleeding the system. I did not flush the system. I have not experienced any overheating issues during normal driving. Prior to changing out the water pump, etc.(e.g. last year), I have done several DE track days and experienced no overheating problems. Do you think their still air in the system? Any other ideas? Thanks for your help.