Ive got a 2001 Monte 3.4L and im having issues with my cooling fan/radiator fan. They seem to be intermittently working. Some days after driving for a while, if im at a red light or idling somewhere the car will start to get too hot. Not quite overheating but hot enough to make the car run crappy. When this happens the only way i can get the fans to come on is by turning the inside air vents to defrost. The heater doesnt have to be on. As long as i got the vent on defrost and air flow the fans come on. But sometimes ill drive all day and i can let the car idle with no issues what so ever. It doesnt make any sense to me....lol

 

I would check the fan relays under the hood. I'm no electrical engineer, but I would think its possible for the relay to get stuck, and a small surge in power on the circuit might cause them to reset and then engage. I would say just replace those 2 relays before you go too crazy.