Newbie with a cooling problem

[Idgie's I30]

I am new to this site so hello to all. I own a '98 I30 and recently the service engine soon light came on and now my car doesn't want to keep the temperature up. What I mean is when Im driving the temp. falls and when the car sits idle it goes back up. Can anyone help me to figure out what it might be? Thank you in advance.

 

[bossbrakes]

heat up

It seems that there is no one with technical background on the I30 site.

The problem seems to revolve around the thermostat.
The I30 warms up at 50 F in around 6 minutes when properly working. Does it take any longer?