I think I have a bad thermostat, and I also think my fan has not ran for the past couple weeks? My water temp gauge is normal when I'm driving, but when I slow down or stop after it has been running for a while, it's buried on HOT, when I start driving again, it goes back to normal, etc. How do I tell what the problem is?

 

So you overheat when idling, correct?

Bleed the coolant system first, you may have an air pocket trapped in there.

If that does not fix the issue, check whether or not the fans are coming on. You should be able to jump the fan switch and have it come on.

When the car is warmed up, touch the lower hose. Is it warm, or cold? If cold, your Tstat is stuck closed and that is the issue. Also, how is the heat in the car?

Here are a couple guides I made for my work. I have just been too lazy to bring them over here for the DIY section. Make sure you check the overheating troubleshooting guide.

http://www.mishimoto.com/how-to-center.html

Most were done on my 99 EX D16y8, so everything should be similar to your car.