Another thing you can do is start the car up with the radiator cap off. Check the flow across the radiator with the engine running and warmed up. There should be a good bit of turbidity/flow at idle. If the flow appears reduced or restricted, it may be a clogged radiator and/or a worn water pump. I've seen a lot of people that buy an older car and the coolant is 25 years old and never been changed & they wonder why the car overheats! Good luck with yours!
Matt B.
This flow test is definitely on my weekend list to do. I've replaced the thermostate twice now and the fluid has been changed 3 times since I bought the car 2 years ago. I've also tried 'water wetter' with some moderate success. The radiator appears fairly new (i.e. less than 5 years old), but I can't say for certain how old it is. When I've visually checked the inside, I can see that it is very clean inside without any obvious signs of rust or corrosion. But, I know that what one sees with the cap off is only part of the story.
What makes this hard to diagnose is that the car doesn't leak a drop of coolant anywhere and I can take it out several times before the temp gives me trouble. The pattern I've noticed is overheating while driving around town with frequent stopping/starting-- even in moderately warm days (70s to low 80s F).
I'll update this post again after I've tested a few more things this weekend. Thanks again for all the helpful suggestions. I'm eagerly awaiting the day when I don't have to worry about making it home or not from the local grocery store.