Overheating...

I have a 2005 montana sv6.  I have replaced the
thermostat, temp coolant sensor, and flush and
filled the radiator.  I have checked all the fan relays
and they click.  I have checked the fan motor with
jumper wires and it functions as expected.  I only
notice the fans come on after it begins to overheat
to about 3\4 full on the temperature gauge.  I even
replaced the thermostat twice thinking maybe the
new one had an intermitten problem.  As the car
begins to heat up about 1\4 way we have heat
from the vents but as the temp approaches the
middle the heat goes away.  Any ideas would help.  
I think I'm down to bad radiator, head gasket, or
something I haven't checked w the cooling fan.  It
did operate for about 1/2 day without overheating
but now it overheats in about 15 to 20 minutes.