There are two things I think of quickly. Others may have more to say. First, check voltage on the wires at that clutch (not at the switch or anywhere else - it must be at the clutch) when it is running and particularly when it is hot. Low voltage can cause such symptoms, but worse yet, it may slip without you realizing it and can cause the clutch to burn up in short order.
Secondly, how many hours are on the machine. Those clutches seem to have a limited life span. Clutch coils can get weak especially if they are partially burnt. You may be in for a new clutch.
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Matthew B 
1972 129 w/hydraulic lift and head light option soon to be repainted
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