350 is overheating in idle/park/heavy traffic

[Bull72]

Can someone provide some insight to this issue I am having? My 1972 blazer is overheating when in drive but not giving it any gas. For example I was in very heavy traffic today and it was stop/go and it started overheating quickly. It will also overheat in idle or at the drive through.

As soon as I start driving though the temperature goes down. Has anybody had this issue before? What would be the suggestions to look at. I take my truck to the same shop and they told me they weren't sure what the issue was. So far I have replaced the radiator, put in a failsafe thermo. and the temp guage is working good. When driving it shows cool and heats up a little when driving long distances. As soon as i am in traffic or park/idle though it just rises.

Any suggestions?