What could be causing a 2000 Cadillac DeVille to overheat when at slower speed

Hello, overheating with little or no air movement sounds like a cooling fan issue. Check the cooling fans for proper operation and speed; also check for any debris that may be restricting radiator flow. All of this is a guess though; for hands on assistance use this link to find a shop near you: www.repairpal.com/repair-shops