Try a higher rated fan switch.

That said I've just put a new one in mine as I have a similar problem with my zetec turbo using cvh stat housing. Mine now come on later but they don't switch off. If I stop the engine let the fans run so the coolant in the rad goes cold then start the engine and let the cool coolant pump into the engine the fans will go off. I'm certain they are on while driving. I've tried a few fan switches.

My next point of call will be to get a infrared thermometer and see what temp the stat housing and sensor is compared to what the ecu says it is. I'm wondering if its a heat soak issue.

After September I'm going to sort the fan control out on the ecu and let that control the fans.