1999 cutlass overheating
 

hi, so 140k on 3.1 liter
new water pump and thermo and flushed cooling system
no water in oil
no white smoke when reaching opperating temp.
overheats and loses anti freeze
no visible leaks fans kick on
ran for 1 hour in driveway fine
daughter uses gets stuck needs antifreeze overheats
any ideas
thanks