Aftermarket fans have been known to wreak havoc on our cars. The blower motor is helping to cool the coolant down when its on. A thermostat won't help your condition. The air dam below the radiator bolted to the core support has to be there. You also need a fan shroud to pull air through the entire radiator surface.
That 15inch fan might not be pulling enough air. The Contour fans will solve your problem without a doubt. The shroud needs to be modified a little for them to work....but not a lot. There are some threads here talking about them....including my own.