With Rocket balking at high speed runs these days, it is time to find and fix the problem.
One puzzling part of this is that Rocket did this when it was young, at around 5,000 to 7,000 miles. It would run fast, then suddenly lose much of its power and slow down by 5 to 10 mph. I don't remember the temperature going up, but it probably did. Also, when I had Lightning, the white 2005 GT, it did this on the Iron Butt ride. On the next to last leg in South Carolina and Georgia on I-95, it went from cruising with the fast cars in the upper 70's to hanging out with the trucks in the mid 60's and running warm. Because I was far from home and had no ability to fix anything, I kept going. After a gas stop it gained its speed back for the last leg.
These are strange machines.
George, (Motovista), the owner of Scooter Parts Co., suggested several things. The water pump was his first thought but I believe Rocket's is OK, as Boris took it apart a while ago looking for the sound that turned out to be the Camshaft. He found it to be fine.
The easier suggestion is to change the coolant using a higher water-to-coolant content. He also suggested changing the oil to a heavier grade. I will make these changes this weekend and see if it helps.