Help, Boxster getting hot!

[truegearhead]

I'll preface this by saying that with the heat index its been 115 here during the day which certainly doesn't help. That aside the car has been heating up beyond half way and even threw the warning light yesterday while sitting in traffic. It will heat up on the highway as well at 70 miles per hour. Both fans are on in fact the car sounds like an airplane trying to take off when its sitting still, the coolant level is great, and it doesn't look like there are too many leaves stuck under the bumper in front of radiators but there is a bit in the back corners. Any ideas what I should be looking for? Could it be as simple as a thermostat, or a coolant flush? I'm driving 400 miles to the Porsches to Oxford car show next week so I need to get this sorted.

Cheers