Yeah, mine shakes at 30 as well, I thought I had to repack the wheels or something. I have no maintenance records on the thing, so one of these days when I get some time I want to just tear everything down....
So you're basically doing the same conversion I want to, weird. What carb are you running? Having issues with the clutch too? I read somewhere else that modding a Sachs will tear up the clutch pretty frequently... Before everything broke, what kind of speeds were you getting?