A dealer "flush-n-fill" is going to run you $200+ so you want to make sure there is no other problem that will require another flush/fill.
MNBoxster's suggestions are fantastic. Complete them first but wait on the flush-n-fill. If those don't fix the problem here are some other things to look for:
1) Check your coolant level at stone cold before driving it 2-3 days in a row and make sure the level is not fluctuating in any way. Also pull up the rear trunk flooring a little around the tank and see if there is any sign of coolant under it. If you find coolant in the trunk or a lowering level you may have a cracked coolant reservoir. It cracks from the back of the tank so you can't see it and it's a common problem. 4-6hour DIY depending on your available tools.
2) Check your coolant reservoir cap and make sure you have the updated cap. It is supposed to help keep your tank from cracking like many have. The last two numbers on the new cap are "01" and the old cap has the number "00". It's $15 at the dealer so you should do it no matter what else could be wrong.
3) Make sure all of your fans are running properly. There should be two up front, one on each side of the bumper, and one in the rear engine complartment on the passenger side. I believe all of them should be running if you get over the 195 degree mark. They should not make any kind of "buzzing" sounds when they turn off either. That's a sign of the bearings wearing out causing the fan to turn slower. Replacement is easy, 30min-1hr DIY depending on which fan it is.
Other questions are what weight oil are you using? Higher weight (15w50 or 20w50), higher engine temps. You should be running in the 195F range when driving at speed and the 200-215F range in stop-and-go traffic when it's about 80-90F outside. Overheat is at 250F so you're not in the "danger zone", but check everything out asap so you're not put in that position.
Good luck!
