Check any balance issues on wheels and make sure they are locked down good, make sure motor mounting is tight, check steering to see if forks are not wiggling around, just some tings I mad sure to check on before I got up to faster speeds.
usually its because the tires are sitting on the wheels unevenly. Almost always the case with cheaper chinese tires. But iv got a few 4" tires that are absoluely hopeless. the rubber is just completely out of round. I guess thats why they say low speed on the sidewalls lol
Well maybe not balancing , but it was one of the first things I figured to mention as it was a general question asked, but as jrmints1415 pointed out, maybe bearings.
You think your engine might just be severely unbalanced? I have one of those piece of crap bicycle engine kits from ebay, my vision goes blurry going full throttle on flat ground .