95 overheating- not intake gasket

I have a 95 Monte Carlo LS and for the past 2 mornings, it has overheated on me. I had the coolant topped off yesterday and this morning the 'low coolant' light came on. Once the antifreeze gets flowing, it doesn't overheat. I just had the intake gasket manifold replaced last year, so it's not that- and there is no coolant mixing with the oil. Perhaps it is a t-stat?! Any suggestions?! I've also had a new radiator put in too.

Make sure you have fluid coming out of the two vents, one on the tower above the water pump, and one on the thermostat housing area. Also, the low coolant sending unit ( passenger side of radiator, 4-5 inches below spout) do go bad. I replaced it three times on my '95. Go ahead and replace the t-stat for safe measures