I've had the same type issues, fine out on the road, get in traffic and watch the guage climb. One thing that I changed was the fan, I had a fixed 7 blade fan and I switched to the mopar 5 blade viscus fan deal-the first thing I noticed as how much more air the new fan pushed at idle, you could feel it standing next to the car. So far I can't say that it has fixed it 100% because I changed it out late last season, but the few hot days I drove it the problem had gone away. I think the difference between the two fans is the pitch of the blade- the fixed was 1.75" and the other was 2.25"(I think, it may have been 2) which means with each revolution of the fan the fan would pull almost an extra 1/2 inch of air through.
Good luck and dont smash your car!
Scott