I had a 1980 model that would overheat (maybe seize) after 30 minutes of hard riding. Is this your problem, or is it just when warmed up? Sounds like it may be running rich. Sounds backwards, but it is a problem when warm rather that cold. I had a Honda CB400T that did the same thing between 6-7k after fitting a 2 - 1 exhaust. I richened the main jets and made it worse. Sounded just like a rev limiter. Does it start easily from cold?