The speed does seem a little more sensitive to me on the slow end of the dial.
We had ours starting to speed up on us and it turned out the throttle assembly had gone bad. Our bike was similar to your problem at first, but the bike would not stop with out shuting the key off. The good part is the bike was only walking speed. I called up OSET and they were very,very helpful. We ride ours in all kinds of weather and water does get on the bike often and this is the only problem we have had. The parts was easy to install, the grip were the hardest part, we use PW50 grips and I reused it from the old one.
Good luck!
Dj