I had the same problem, I have a 2002 Windsor with a 350 HP 8.3 ISC.
Jeff, do you have the thermovalve controller that is mounted on the radiator??
It looks like this & has 2 #6 hydraulic lines to it.
Sauer Danfoss Thermal Valve 553/1/09857/210**: nwrvsupply.com
If so yes it can be tested so you can tell if the valve is working correctly and/or the motor speed controller isn't working correctly.
If you have the thermo valve go buy 2 plugs and a union.
Take the lines off and plug both lines and see if the fan runs wide open.
Then put the union in the 2 lines and see if the fan runs at ideal.
If does this the motor speed controller is working as it should and chances are your thermo valve is bad.
I purchased my thermo valve from White House Products, cost was ~$265 including freight and exchange rate.
The part # was 553/1/09857/210, took ~1 week to get after ordering. There should be a tag on your thermo valve with part #.
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