For the first 80-120 miles (1.5-2 hours) it will run 1900-2000 rpm in 8th gear and go 72 miles per hour. Then it becomes consistently sluggish and only achieves 1500-1600 rpm and 60-65 mph. When I stop and turn it off for 15-20 minutes and restart, the problem goes away. Only to rear its ugly head in 80-120 miles
That sounds fuel delivery related. Fuel filters, fuel lines, or transfer pump are suspect. Possibly the fuel pickup being blocked by something floating by sometimes.
