this is actually a cool feature, and it can be turned on/off in the PRV preferences.
if your adding in notes or making edits on the fly,the prv will stop scrolling for you because it thinks you want it to.
like dan said,just right click anywhere in the prv to reset it.
this can be (like I said turned on or off)in the prv settings.
which one ?
(not at the daw right now,but I can get back to you on this. :)
if you find this wont take care of the problem,look on your keyboard theres a key called "scroll lock"