1997 Chevy Truck engine revs slowly after driving at high r

A little history on the truck: I have had truck for 7 years, towing 7000 lb camper, largest aux trans cooler with factory one in radiator. In 2005 I have replaced fuel pump twice, and 2 weeks ago I replaced the fuel pressure regulator and plenum gasket.
The problem: after driving at highway speeds between 20 and 30 minutes, the engine starts to go up to 200-210 degrees, normally ran 180. After running for a while, even at 180, the engine revs very slowly from 3000 to 4000 rpms and seems like something is holding it back. If I kick the engine down, and the motor goes up to 3500 rpm, it will hold there and crawl to 4000.
What are your thoughts? Thank you.