With a new season comes a new issue... overheating

[MagicalAnimal]

Good morning,

So it's officially fall up here in New England and of course that means my Alero wants to give me some more grief.... I've found I get one new problem each time the seasons change LOL.

I have a 2000 V6 with about 147K(is that the record? LOL). This morning I was driving to work (about 60 miles, all highway) and I made it almost to the end of my ride where I had to go through a toll plaza. After going through and working up the ramp to the next stretch of highway I hear the dreaded alero "DING DING DING" and it's the temp gage up in the RED and I realize the air coming from my vents is COLD even though the heat is on full blast. Luckily I picked up speed on the highway and the temp came back down and heat came through my vents and everything seemed OK. My work is right off the highway so I didn't have to idle really any more in traffic but as soon as I got off the highway the heat did stop blowing hot and only came out cold again. I parked it and came into work but Im going to go out there on my lunch break to see what I can find.

I am not really sure what the issue is right off the top of my head because it was so random. I don't think it should be the LIM gasket issue because I had that fixed about 1.5 years ago and I definitely didn't smell anything like I did when the LIM actually went bad(Maple Syrupy Smell). I drive a lot all the time and yesterday drove a good 200 miles and she was acting perfectly so I am really confused as to what is wrong. No driving or startup issues and I had heat my whole ride in.. granted I was going about 70-80mph so the air was being forced into the car, so I don't really know if it was like this when I started it or if something happened while driving. Any insight into what I should look at/for when I go back out there later would be much appreciated.

Thanks!

****** Additional Info ********

So I went outside on my break to try and dig further into this. First I checked the coolant to see if that had leaked and caused the overheating and the surge tank seemed to be fine. Slightly below the "cool" line, but I haven't really checked it in a while so it's probably just my fault for not topping it off in MANY months... don't think leaking is the issue because there was a good amount in the tank and it was the proper color (Orange-I still use the Dexcool). Next, I thought I would purposely overheat the car so I could check the fans to see if they were engaging and if not, to see if they came on when I turned the AC on (I read on a different post about another possibility being the ECM or PCM or something not turning the fans on) but as soon as the temp needle rose slightly above the 1/2 way mark on the temp dial the fans in the motor kicked on and the temp came back down to exactly 1/2 way, which is where it normally sits. Both fans engaged and ran fine. I then left the car still running/idling to see if it would overheat but by the end of my break it did not. I only had 15 minutes so maybe it just wasn't enough time. I'm going to take her for a drive at lunch to see if I can make the problem happen again. IN the meantime.. and insight is appreciated. I love knowing what's wrong with my car before having it fixed so I know what to expect.
Thanks!

[jr6078]

Sounds like the thermostat stuck closed, but is now opening properly again. I'd keep an eye on it for a couple days, but it will likely happen again, and it will probably stop opening altogether in time.

[MagicalAnimal]

So I went out on my lunch break and took the car for a drive to really heat the motor up. It never went above 1/2 way while driving so I came back to the parking lot and sat in a parking space with the car still in drive with my foot on the brake. The temp needle started to rise and at 3/4 the fans kicked on and brought it back down to 1/2 way (normal)... so 2 times I tried to recreate the issue and couldn't. I am definitely going to change the thermostat tomorrow and flush the coolant. Is there anything else anyone thinks it could be?