my 1990 300 SE is overheating when i stop in a stop light. It is very hot in san jose this week reaching 90 or so. When i drive, the temperature stays at around 80 but 5 minutes into the highway starting moving to 85 or even 90 and exiting freeway idling at a stop light reached almost 100. We checked the coolant and it was full so i wonder what else it could be. In the cool days it runs well.