Could be any number of things, to be honest - fuel problems, spark problems, even binding brakes [1]. Tell us a bit more about the problem and narrow it down a bit. Does it start well? Idle OK? What is the problem when it loses power - gradual lack of urge, or a misfire? Exhaust smoke? Have you checked plug gap, tappet clearances, compression? Good or poor mpg?
[1] To illustrate: I had a mate with a Honda 250 that wouldn't go above 50 mph. Every time he got up close to that speed, the bike would start to struggle, almost as if the brakes were on. He had checked everything, and spent pounds on replacing possibly faulty components. The clue was that his front brake went bad at the same time. On inspection, the front pads were glazed. It was a cold British winter, and the bike was wearing those handlebar muffs. I told him to go for a run and, when the bike started struggling, to straighten the fingers of his right hand. Sure enough, the bike carried on accelerating up to 70 or so. The wind had been blowing the muff back onto the brake lever, creating more drag the faster he tried to go. The lesson we learned from that: assume nothing, and rule nothing out.
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