Weather, outside temp, etc... make no difference if you have a bad fan. If your fan is going bad, you will see problems with overheating in stop-and-go/idle. It's a mechanical defect within the fan, and it can fail at any time. Some have died as early as one day... mine took two weeks and 680 miles. Regardless, if you are seeing overheating, get to the dealer and demand a replacement aux fan. If they question you, reference this board and have them cann BMWNA. BMWNA is aware of the problem, even if they can't issure a service bulletin yet. They can call your dealer and tell them what's up.