2004 Malibu Slightly Overheats

I bought a 2004 Chevy Malibu about 4 months ago and I've had nothing but problems with it. Here's my current issue with it...

The car used to SLIGHTHLY inch its way up on the temperature gauge -- I only observed the temperature gauge reach halfway between the halfway mark and the red mark -- when I'd be idle in highway or city traffic. So I thought it might be the fans not being turned on. I turned on the A/C to turn the fans on and that fixed the overheating in idle problem. However, 2 weeks later the problem persisted, not only when I'm in idle, but when I'm driving too.

The other day while I tested it in my parking lot in idle mode, with the A/C turned on, the car's temperature gauge once again reached halfway between the halfway mark and the red mark. I turned the car off because I was scared to see if it'd actually completely overheat. When driving over long distances, the car's temperature gauge goes barely over the halfway mark and it will then work its way down barely below the halfway mark; it repeats this process. There's some kind of heating/cooling issue. Any help or suggestions?

Oh, and by the way, I have PLENTY of engine coolant.

If it's not anti freeze, I would next check the thermostat, pretty minor job.

The thermostat in my '04 Maxx was replaced over the holidays. I had no fluctuations with the temp guage (stayed right around the halfway mark), but the "Check Engine" light had come on.

Now, once in a while, the temp guage fails to work at all.

These cars are full of electrical gremlins, among a slew of other problems.