Probably not rebuilding the library just updating it in which everytime you unplug it it updates and it does take a bit of time depending on the size. The jerking can be cause by many things before I went to gigabit I installed a NAS and that worked great even at 10/100 I even pulled some native blue ray movies no problem. I would say at this time it would be something in your computer slowing it down. Either try to install a movie on the Hubs hardrive and see if it plays there. or you can connect a usb drive straight to it to see if it plays from that. If all is good there then it is either network or computer
I am currently and always have had a problem with the ISO locking up shortly after starting several reboots can correct the movie most of the time or I go to preview mode and play it from there kinda a pain.