[DonC]

I vote for there still being some air in the system if everything was working properly before the switch was changed out.
My procedure is to fill the system cold and then jack the front of the car up as high as I can get it and leave it for about 20 minutes. I can usually hear the air bubbling out as soon as I get the front end up. With the front end still up fill until the level is just about to the separator plate in the expansion tank. Reinstall the cap, let the front end down and run the car through two or three heat cycles. I can watch the temp gauge come up to thermostat temperature, hesitate for a minute or so and then slowly climb to the fan switch temp. At that point the temp will drop back down to near thermostat opening temp and just wander back and forth between the two temp limits. Check the system after each heat cycle and refill or remove coolant as needed.
Don't forget to keep the RPM's up to at least 14 - 16 hundred so the water pump is working at a level sufficient to circulate the coolant. If you're careful you can leave the cap off the expansion tank and watch for coolant circulation as the temp gets near the thermostat opening point. Once you've verified circulation recap the tank and continue through the fill cycle.
DonC