Ron,
A first question I always ask is "what changed" recently. If something was changed, I would probably check to make sure there is not an air bubble trapped in the system. Jack the front end up, putting the filler/expansion tank (if you have one) at the highest point. See if there are bubbles to get out. It can absolutely cause a strange heating situation you've described before. It happens in FEs often too with low expansion tanks. Also, you see this situation occur when you go from distilled water only (much better cooling property) to anti freeze (actually worse than straight water, just raises the boiling point), and let say by increasing the ratio of AF to water actually see a notable temp difference for one. I've measure up to 10-12 degrees difference before. Also, if you were always running your car with a 160 degree thermostat before, typically running the motor a little on the "cold" side (not always a good idea) by simply installing a 180 thermostat and running too much anti-freeze actually see the car run notably warmer in traffic on a hot day as compared to before. Also, in some cases seen brand new thermostats stick closed periodically for whatever reason. I'm not saying this is your problem, but I have seen this condition and scenario occur in Cobras before with insufficient cooling fans; in traffic situation on hot days. If it continues, and if you can't solve it easily, you can always test the thermostat in a pan of boiling water to see if it opens/closes or not and try draining the system and replace with distilled water. If the straight distilled water works better, I usually add 1-2 bottles of Water Wetter in Sacramento instead of anti-freeze; then adding
antifreeze back in again as winter approaches again. Just a few experiments if you can't find anything else going on. Good luck.