bluox wrote:

If you go to Cat get screen gasket and O-rings for filter.
You will need 10 us gallons of oil a SAE10W TO-4 spec.
While you're under tractor pull the torque screen,clesn it and see what's in it.
Bob
 

I got the oil out, but where can I find the torque screen? 

Behind the key shaped cover on the back/bottom of the transmission case.

Actually the screen is on the front of the transmission case right below where the drive flange goes into the torque converter and under the filter housing.
If you go to Cat have them price you an L&M guide.
Bob

a dirty screen wont help with overheating either. part of the job of the oil is to also carry heat away from the transmission parts too.

I installed a new water pump today and had the radiator cleaned. I also replaced the transmission filter and changed the milky trans fluid.

After a 15-minute test ride, it looked good, with the temperature gauge in the lower end of the green area. However, 10 minutes later, I noticed that the coolant temperature began to rise. Five minutes after that, it was in the red area again, dang!

Now, the weird thing is that when I started the engine after turning it off to check the coolant level, the coolant temperature rapidly declined. It went from red to normal in around 20 seconds. The engine was only off for maybe 2-3 minutes

Could this be an airlock, or is it a symptom of something completely different?

Should I purge the cooling system, and if yes, how?

Thermostat been checked? 

Yes - it’s brand new and I tested it before I installed it.
 

Wouldn't hurt to replace the guage too.

I cross checked it with an infrared thermostat. Unfortunately the gauge is right, the d4d is overheating for whatever reason.

The coolant temperature drop was also real, it dropped from around 220F to around 180F.
 

Take thermostat out and try it.