So I went out today and checked all the items suggested here; screen, radiator, HST fan etc. The fan was fine. The screen and radiator were a little dirty, but just barely so I cleaned them both. Last night I dug through the manual and realized I was wrong about the rated RPM. I thought it was 2500 but the manual indicated 3200. I took the tractor out and ran the bush hog at 3200 rpm. When cutting the rpm dropped to around 3000 under load. I only used about 1/2 to 2/3 throttle so as not to be so fast. It ran pretty good. I noticed that when I got a little too fast it did hit the red line but dropped back down when I slowed down the driving speed while cutting.
Also, I noticed that at about 1/2 throttle the temp would get high but not really hitting the red. When I would hit thick stuff it would almost redline but just barely and I mean barely stay out of the red. When I would get past the thick stuff and into the thinner stuff the temp would drop back down. It was cycle as I made passes around the field. I will also say that yesterday when I kept over heating it was near 90F with high humidity. This morning it was around 75F with lower humidity and a breeze. Could be a factor, maybe not, worth mentioning though.
All in all, everything seems to be running pretty good now. Lesson learned - ensure screen/panel/radiator are clean, run engine at 3200 rpm, drive about 1/2 throttle.