My dad's 04 sporty did this a few times on me, was fine until I got up to a faster speed. We found trash in the tank. disconnect all the lines on the tank (making sure to turn off the petcock) and then remove from the bike. Dump the gas into a bucket pouring through a filter to remove the trash. Remove your petcock and clean the filter, then cover the petcock hole with your finger or a rag, and pour clean gas into the tank; shake it and move it around and then pour the gas back in the bucket through the filter to keep the trash out. Do this until no trash comes out then put the clean gas back in the tank after re-install.
Clean the Main Jet, Float bowl, Check the float level. (May be overloading) Peek inside with the Air Cleaner off to see a good stream push when you give it throttle. Change the plugs. (Plugs can work up to a certain PSI before cutting out) This may be the problem as well. Check hoses for cracks
Does it rev freely in neutral but just flattens out at 60?
I had a throttle cable that vibrated loose and decided to adjust its self where I could only get about 1/4 throttle. Took me a bit to figure out why I was running at prius speeds. Just an idea.