I was wondering if anyone else has encountered this. When I'm on the highway my temp gauge constantly moves. It will hit just over 190 then will drop down to about 125 then back up to 190. It'll sit and idle and not get too hot even plowed last week with no problems. It didn't start doing this till I changed the thermostat

 

Return the aftermarket thermostat & buy a Cummins. Keep the O-ring for your troubles in case the Cummins seal lets go.

 

I'd go with thermostat on this one.

Get the Cummins stat from a Cummins dealer for the best results. Mine just cost $62 USD.

 

That's what I have. I got fed up and went through the dealer. This one cost me $70,but thought it would help. It is better than the aftermarket one but still not as good as the original

 

thermostats do fail even when brand new, I worked on a dodge car a few yrs ago and I went thru 5 !!! before I got one that worked properly, the auto store kept taking the failed thermostat back.

typically the OEM stats have a better sucess rate, if your seeing that much temp swing it is bad right out of the box,...
or you have a large pocket of air trapped under the thermostat, to confirm just remove coolant sensor (right beside thermostat housing) and fill rad until coolant comes out.

or your coolant level is too low

 

As nickg mentioned, check the coolant level. That will cause erratic temperatures while driving. Also, what is your in cab heat doing during this fluctuation? If the coolant is up then I'd lean towards a thermostat too but only thing is that thermostats usually dont cause this problem. They usually tend to not close all the way which causes low operating temps in cold weather or they dont open at all which causes overheating.

 

That's the thing the heat in the cab seems to be pretty stable. I'm starting to lean towards maybe an air pocket, I haven't had a chance to do anything with the truck yet, but gonna make some time on mon to mess around with it. Thank you to everyone for all the input, it's much appreciated.