radiator upgrade

06 rt 5.7 No mods at all other than a drop in k&n air filter, had a tune up done on it (spark plugs, flush coolant, etc.) because i just purchased it not long ago...drove it around all day and no problems until I went to the store to buy pool salt, my wife was sitting in the car letting it run, I put 6 40lb bags in the back and the car overheats before I leave the parking lot. A former tech at dodge that came to see if he could help said it was the radiator because the coolant would boil then stop then boil again after a few minutes. I let it cool off and took off down the road and no problems.

I looked for leaks while the car was cooling off and I couldn't find any. I had the thermostat replaced when the tune up was done. I noticed that the fan always seems to be running on high when it's at normal operating temps, ill have dodge look into that. I guess it was most likely air in the system and when I put weight in the trunk the air pockets moved toward the front. I'll drive it around tomorrow and see how it runs and if the problem persists I'll take it to a shop. Thank you for the reply