I have an Avanti Autopower and my back wheel locked up once on me too, during the break-in period as I came down a hill.... I put more oil in the case and it never locked up on me again ( knock on wood )....The bikes are shipped to the dealers without tranny oil...I feel that the dealer was in a big hurry when they
" prepped " the bike and maybe never put enough oil in the transmission case to begin with.... my advice is that if you buy one, before you ride it.... pull the drain plug , empty the tranny case, and then put in EXACTLY 350 ml of 10-30 wt. oil...that way you know exactly that there is the proper amount of tranny oil in there to begin with....also make sure all the engine bolts are screwed in tight...and don't max out the engine during the break-in period.
I have almost 1,000 miles on this Avanti already( had it almost 2 months ) and I like the bike, it is my major form of transportation right now, my car is getting a big rest sitting at the curb all week !