I wonder if your fans are not working on high speed. It sounds like your tank is not the issue and stop-and-go traffic in high temps with the AC on will really give your cooling system a work out. Did you note how high your gauge was running on the dash?
1) Maybe post a thread titled "Want to borrow a Durametric in the DC area." If you were local, I'd be glad to help and I bet there are forum members in your area that will if you can find them.
2) Search some of the threads about the fan resistor and relays, like these:
http://986forum.com/forums/boxster-general-discussions/46404-one-my-rad-fans-has-stopped-working.html
http://986forum.com/forums/performance-technical-chat/54024-radiator-cooling-fan-resistor-values.html
Keep a gallon of distilled water in the frunk, just in case....