Did the problem start suddenly or gradually? Does it actually over rev? Does the speed seems to be normal and predictable while the engine is still cold? If it is simply surging up and down, and not necessarily over-revving, sounds like a lean mixture on the carburetor. Try giving it a little choke and see if that smooths it out. If that is the case, I would check fuel supply and probably suggest that the carburetor needs a good cleaning. It could also be a governor shaft and/or linkage binding, but generally that makes it go way over top governed speed, as Don suggested.
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Matthew B 
1972 129 w/hydraulic lift and head light option soon to be repainted
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