Usually warms up to right before the 190 mark...

stays down around the second line on the temp gauge...

 

When im not towing mine stays right below the 190 mark all the time. It's wierd though, because when I tow, it tends to drop down alot colder at times, i'm not sure if thats the fan doing its thing or what?

 

Thermostat stuck?

Runs fine...

I really got on it on the interstate and got it to 190 and it held there... (80mph, 700*F)

 

I got the same problem the only time mine hits 190 is when I got a big loud on or I am running it hard. I just keep running it. thats the way I bought it.
It runs fine.

 

If nothing has changed and the temp used to go up to 190 but now it doesn't, I'd say something is up. Why would the same truck start acting differently all of a sudden? Maybe the sensor went bad?

 

Would timing have an effect and I just didn't notice it?

 

It seems to stay around 155-160 then after about 10 miles on the freeway it will go up halfway between the 150 and 190 mark...

Oil level is fine and coolant is good...

 

The Coolant Temperature Gauge Will Fluctuate On Diesel Engine Equipped Vehicles. It Is Influenced By Vehicle Load, Outside Ambient Temperature And Extended Idle Time. This Function Is Considered Normal

Gauge Fluctuation Will Be Noticed Most Often Under Steady State (highway) Driving Using This Thermostat (p/n 4796869). Please Ensure That Your Customer Is Aware Of This Note Prior To Releasing The Vehicle.

 

More than likely the rubber sealing ring has come off and is holding the thermostat open. Seems to be a poor design and happens quite often.
I've found the sealing ring is unnecessary and just take it off and trash it. The thermostat works just fine without it.