My guess would be a sticky thermostat or partially clogged radiator.
At one point, Dexcool (deathcool) was the recommended coolant for this bike. If your coolant is red, or you see red mud on the cap, overflow tank, etc., a full flush and clean should be done. Then add silicate-free green antifreeze mixed with distilled water. But even without the deathcool problem, any system can get plugged with scale and residue over many miles.
Repair manual can be downloaded free, it would help you a lot.
The thermostat is under the gas tank in an aluminum housing, just follow the hoses. For testing, you can put the t-stat in a pot of water and heat it up. Observe how well it opens/closes. If it hangs open when cool, or doesn't open fully when hot, replace it. Ebay is the first place I check for parts.
While you have the t-stat and hoses removed, you can flush with a garden hose. Flush in all directions of flow, this backflush can remove suspended gunk (remove the drain plugs on the radiator and engine). A long soak with a radiator flush chemical can help, but those have been made weaker in recent years. Distilled white vinegar does pretty well, you could fill the whole system with it and run until good and warm, even ride a while, then flush with clear water. I like those brass, twist open nozzles that will fit inside the hoses.
Any time your bike overheats, getting up to 25-30 mph should cool it off if there's no major mechanical or flow problem in the system.