Coolant over flow gauge normal

[unboxed]

I recently bought a 2001 Boxster S. I've driven it on several trips over 4 hours each way and it did great. My wife drove it on a short trip that was two hours each way. On her return trip, the coolant light started blinking slowly but the gauge remained just above 180 which appears to be normal. When she pulled into the driveway, coolant over flowed under the car, not from the tank in the trunk. It has the original cap with -00 which some posts indicate may be a problem but if the cap is the problem wouldn't it have over flowed at the cap? Why would the gauge read normal and it still over flow? Any suggestions on where to purchase Porsche coolant other than the dealer? Any help is appreciated!