I've recently done a few hours of highway travel on my Glee, and I noticed I kept slowing down. ... as in, I'd throttle up to 65mph and hold it there. Some minutes later I'd notice my speed had dropped to 55-58mph, so I'd give it more throttle get back up to 65mph. A little while later I'd be slowed down again.
I found it very hard to hold a steady pace without a lot of monitoring the speedo/ GPS and correcting. It happened in both directions, so I don't think it was related a long climb. Never noticed this on my Vee, only the 650.