I have a pa50I and sometimes when I was riding it, it would only go about 15 mph...then if I hit rather large bump or hopped of a curb, it would all of a sudden go up to 18-20 mph. Well yesterday when I was putting on my ramp plate from the pa50II, I noticed there were little metal clips that held the weights. Not really sure why they were there, but I bet that they were catching my weights and not letting them slid out...but when I hit the bump it would knock them free. I didn't put the clips in as I really can't see why they are there. That might help you out maybe if you took those out?
Just a suggestion.