1996 Chevy Lumina Not overheating but running hot

It could be several things: old/low coolant, sticky thermostat, water pump going bad, radiator/water jackets getting clogged, cooling fan not working, loose serpentine belt, bad hoses etc.
Start by making sure your coolant is at proper level and in good condition (you can buy a coolant tester for $2 at Wal-Mart). If the level is low, add proper coolant to your radiator (engine cold!!) and your overflow tank. If coolant too old, have your cooling system serviced (new coolant, radiator flushed etc)
Also make sure your fans come on when your car starts to get hot: open the hood; when the temp gauge starts to go beyond the middle, check that the fan located right in front of the radiator comes on. If it doesn’t you have your problem.
If both the coolant and the fans check ok, I suggest you just go ahead and replace the thermostat; a new one only cost $5 and even though it may not be the culprit today, it may save you some problems in the future.
If it still fails to fix your problem, we still have a few suspects on the list: water pump, loose serpentine belt (either the belt has stretched or the belt tensioned is worn), bad hoses, an internal engine leak.

Start by checking your coolant and your cooling fan. Post back if you need more help.

Thanks for the help. I checked my coolant (I had it flushed a year & a half ago) and it was low. Seems to be ok right now, but will be replacing the thermostat anyway because the car sat for a couple years before I bought it. Already had everything else checked out on it when I bought it. I appreciate the response! I hope this fixed the problem.

No problem.
If your coolant was low you could have a leak somewhere, in which case it’s only a matter of time before your car starts overheating again. You may want to inspect it visually (or have it inspected): check the hoses, the radiator, the water pump, around the intake manifold, the thermostat housing.
If there is no visible leak, have the system pressure tested; it will tell you if you have an internal leak. You may also want to have the radiator cap pressure tested. If the cap is old it may no withstand the 13-15 psi of pressure and leak coolant. If you still have the original cap, you may want to simply replace it ($5 at your autopart store).
You’re doing the right thing by replacing your thermostat.
Let me know if you need more help.

Well it hasn't over heated yet, just running hotter than usual. The temp gauge shows 100-260 and usually it run at about 1/4 of the gauge or about 150-160. Then I noticed it would occasionally start going up to about 190-200 when idle and go back down when I started driving. I hadn't changed my oil for about 5 months and it was about 1 quart low so I changed that and they said there was no fluid in the oil. I posted my question here and checked my fluid and it was low, below the cold fill line so I filled it up (accidentally to the hot fill line). Today it got to about 170-180 and then went back down to the usual temperature it runs at and never went back up, even while sitting idle and then driving again like it did in the past. Like I said, it hasn't overheated by any means, just started running a bit hotter and the coolant was low. I don't know if it is an indication of a bigger problem or not, but I want to find out before it does become a bigger, more expensive problem.

Hello there,

150-160F is not normal operating temperature. Your engine should be running between about 195-210 (close to or at the middle of your temp gauge). Your thermostat is calibrated to open at 192-195 F.
I would start by replacing the thermostat; it sounds like it could be stuck open. Quick question: when you turn the heat on while driving, does it put out really warm air or just luke warm/cold air?

Let me know how that goes.

It puts out warm air. Not overly warm, just what it should put out. IT has been running at that 150-160 ever since I got it though, but it had been sitting for awhile.

Again 150-160 is not normal operating temperature. The fact that your gauge shows 150-160 doesn’t mean it is the actual temperature though. It’s possible that your engine runs at normal temperature (around 195F) but your coolant sensor could be bad, or your gauge itself could be off resulting in the gauge showing only 150. You could find out by using an infrared thermometer and checking the coolant temperature at the thermostat housing, (engine idling at ‘normal’ temperature) and comparing the temp on the thermometer to the temp on the temp gauge. You can buy this type of thermometer for $35-40 for the cheapest ones. You could also troubleshoot your coolant sensor by checking its resistance.
Since you said you were going to replace the thermostat, let’s do it and see what happens.
If you feel like tackling this yourself, it’s not a difficult job. All you need is basic tools (wrench, sockets, open wrench, a drain pan), and a Haynes/Chilton manual.
Quick tip: There’s one way you can confirm a thermostat is stuck open: turn on your car when the engine is cold; grab the upper radiator hose with your hand; if you can feel liquid flowing thru it right away, it indicates a bad thermostat. It’s not a very accurate test because ‘feeling’ thru that thick hose while the whole engine is vibrating can be a little hard, but it may help you to confirm a bad thermostat nonetheless.

We are going to try the thermostat in a week or so, I don't have the money until then and I will get back after that. I appreciate the assistance on this, it is much appreciated.

No problem.

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Good luck