Yep, sounds like the temp sensor then. Make sure you put a little teflon tape around it before you re-install it as it may leak. It's not in the easiest spot to get to but can be done. Of course when you pull it you will loose coolant and need to refill/bleed the system.

There may have been a class-action against GM for the Dex. Never really looked. At 75K I would def. flush and refill the cooling system.

Here's my thoughts on Dex. If your cooling system is leak-free and does not have a BIT of air in it and you change it often then Dex is fine and will not cause issues. Where you run into issues (especially on these cars) is when people read the sticker on the pass. strut tower and don't change it until 100K+, or when you develop a coolant leak due to the LIM or elbows and introduce air into the system. When Dex meets air it doesn't like it and turns nasty and will clog everything up from heater cores to rads and attack gaskets. If you take the cap off the rad and have some reddish/brown junk on it then it's time to flush/fill.

I personally went w/ the normal green coolant when I dropped my new motor in. I had a new rad. and heater core go in at the same time so I figured I would avoid that issue altogether. Luckily the used motor I got was well maintained and showed no signs of the Dex going postal in it. Even when I installed the new LIM gaskets it was clean. Flushed it and away I went. Been fine ever since.

Also, don't mix Dex w/ the green stuff. Once again bad things can happen as they don't mix well if at all. It's totally up to you what you want to put back in, but from my point of view I'd rather run green and just change it as it should be and not worry about it eating gaskets or clogging things up just because I got a small leak in a coolant elbow or something. My temps didn't change either so its not running any hotter/cooler than with the Dex.

In the end it comes down to personal preference, your maint. schedule and the condition of your cooling system.

Also, when you go and bleed the system do a search for that here as well as these cars can be a pain to get the air out of completely. I bled mine 3 times before finally getting all the air pockets out. See the brass bleeder screw on top of the thermostat neck? That's what you use. There are other tips on this site as well.