Question on 1995 Lumina sedan
 

I have a 95 Lumina Sedan with a 3.1 liter V6. It's been a good car and I use it for daily commutes to work. Recently, I've noticed the temp gauge has been going beyond the halfway mark and approaching but not entering the red zone. By cutting back on the A/C, I can lower it a bit, but not to where it should be. Here's what I've checked so far:
1) The coolant is visible in the filler tube at the radiator so it is up to the top
2) The reservoir is nearly empty but still has some coolant in it
3) There are no coolant leaks that are visible under the car
4) The cooling fans work fine
My question is could this be a partially failing thermostat or could there be another problem. I've read that getting to the thermostat is complex - removing a crossover tube and possibly a heat shield too. Does anyone have a suggestion?
Thanks Larry