LeSabre 2000 Overheating

Coolant overflow gets more coolant; radiator coolant level low.
After I put water in radiator, temperature gauge 50%-80%. Sometimes no heating (inside the car), sometime with heating. No pressure in upper radiator hoses connecting radiator and engine.
No white smoke; no coolant in oil.
What's the possible problem? thanks.

If I had this problem, I would look to replacing the thermostat. Sounds like it is not opening up sufficiently to let the coolant circulate, resulting in overheating.