Alrightey, so about two weeks ago, when all this raining started, my car started misfiring. I stopped at a local Autozone and had them read the codes, and my car pulled up a cylinder 6 misfire. Couldn't give me any insight other than it probably needed a tune up. (it's Autozone, I wasn't expecting much.) On my father's suggestion with him being the previous owner of this car, I changed the coil, since I had noticed before that it would give a random misfire then go back to itself. While the car ran smoothly after warming up, it didn't stop the misfire. Changed the plugs, and it stopped for a week or so until again this morning... with the rain... it started to misfire again. I stopped and let it idle for a while, and it smoothed out. It misfires until it's warmed up, then the problem goes away until it sits overnight.
The difference between this time and last time, is now my thermostat gauge is fluctuating. When I go to take off, it goes down, then back up where it's supposed to be. My first thought was that it may be stuck open, but I can't say for sure. This is just a base model Sable. Nothing special, but it works for me.
I'm not a complete novice with cars, so you don't have to worry about confusing me. I'll definitely know what you're talking about. Just any suggestions or ideas would be helpful to me at this point.

 

You need to buy your own code reader (a cheap one, $60 USD). Even Walmart has them on sale occasionally. You need to check whenever you want without going to a store.

Changed plugs and it stopped for a while, huh?

Which engine do you have? Vulcan or Duratec (Sitting in the driver's seat, looking at the engine, the intake manifold tubes make sort-of a V for a Vulcan and sort-of a D for a Duratec.)

When it does misfire, determine that it is indeed #6, then look physically for a problem. In particular, look for water seeping from (a channel in) the intake manifold.

This is only a guess, but I suspect leaving it off for overnight allows the water to gradually seep out and accumulate. After you start, the engine warms, and the water evaporates away.

Also, accurately mark the level of coolant in the reservoir after the engine has been cold for a few hours. Don't add coolant. Use the car. Check the level the next night. If it's lower, the fluid going somewhere.

I hope you already saw to it that there was sufficient coolant in the engine. Your temp gauge problem makes me suspicious.

 

Yeah, the temp gauge is a little wierd. Make sure your coolant situation is good before doing anything else. Its probably unrrelated, but may factor in. Have you tried swaping coils? Assuming this is a COP engine, you can take the new (presumably good) #6 coil & swap it with another you know to be good. You can rule that coil out anyway. As well, check the wiring from ECU to the #6 coil. If the coil checks out, swap it back then swap the wiring with another if that's possible. You just have to start ruling everything out until you find the culprit.
Good luck,
Gator