I've been to the jemsite you mentioned, quite helfull but there ain't any adjustment to make the arm's rotation stiffer ... they suggested using a nail polish and painting a layer of the polish on the arm holder and the arm itself to make more friction between them.
Quite funny but I'll try it and hope this solve the problem, at least periodically till the polish layer is gone , then I'll have to do this process again.
But I believe some arms were manufactured to be stiff in their rotation on purpose (I can't believe that Vai uses this cheap nail polish trick , do you?!), I hope that any of the guitar freaks here knows a name or a model for a stiff trem-arm.
Thanks for all replies dudes :D
