Can I ask a couple of questions...?
You said it loses power? ..You mean the engine slows down or the engine revs, but the bike slows down? Does it only lose power when going up a hill? Not on flat ground...? It is only 1 of the 2 you bought correct? Does/did this one get used more or less than the other one? Do you cross water often, or DEEP water at all?
It's hard to explain. It's like the engine slows then picks up It's like it's cycling. Yes it only loses power on a hill. And yes it's only one of them. It's funny my son rides his hard runs it through mud and water and it runs great. On the other hand I ride my fairly slow and don't ride in mud (unless there is no other way to get where I am going LOL) or run it through water. I haven't had it in any water.
My son and I have ridden about the same miles but I just poke along and look at the sights.
I am not seeing the machine in person, but from what you are telling me, I don't think it's a clutch problem. Any chance you have gotten some contamination in the fuel? If you are riding easy most of the time, then the only time you would be heavier on the gas was when going "up hill", so that may be the only time the contamination shows up or causes problems. I would try draining the float bowl on the carb, and adding some SeaFoam additive (or something similar) if you don't want to drain the tank as well.
Hope this helps.....Good Luck!