As to the slowing down, it is a little unclear if the engine actually pulls down or if just the blades slow down. If it is just the blades slow down, I would check the belt system for wear and proper belt tension. Check the clutch as well. If the clutch is slipping, it will get really hot when that happens. Otherwise, there may be some adjustment off, like others here have said.
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Matthew B 
1972 129 w/hydraulic lift and head light option soon to be repainted
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