my guess is that it is running lean and causing it to overheat..maybe sucking vaccum from some where ? or just jetted too lean ...what you describe does sound like a partial sieze..but I am no expert
I have had a '68 and '69 Kawi F3 175 and those things would run hours on end no problem. (watch Cycles South. those guys ran them all the way to Baja) and my '74 TY175 will do the same. What you are describing is not normal by any means. Something aint right.