Detroit Diesel Overheating on Mountain Grades

[Newell Attack]

My immediate issue is a tendency for our engine in the classic newell coach(6V92T) to overheat in a hard climb. About 4 months ago it started and it seems to be getting worse. I have had the radiator power washed twice. Still, towing the car on a steep or long climb, it will heat up. I have always been able to nurse it along and keep it within the operating temperature. I don't think I could do a mountain climb now without letting it cool. Before, it would climb a grade and barely move the gauge. What would you check first? Thought I might get a little education first before I take it in for a checkup. Thanks, ..Jeff