Try all posted above... double check that you didn't get a bad thermostat (put in pot of water with a meat thermometer and make sure it opens at 180...)
Possibly and air pocket like stated above..
x2 on checking the radiator cap..
Are you sure it's your coolant fan that's coming on? You're A/C is off, right?
Are the coolant fans coming on at the right temperature? Possibly a bad sending unit.
When is the last time it was flushed? Possibly blockage? If nothing else works you may try a reverse flush in case something is clogged...
Worst case scenario, check your head gasket... is it running rough? losing power? coolant (foam) in the head when you open the oil fill cap? Oil in the coolant?