are you playing back full BD ISO's of 40+ gig?
I set up my mates Mederer with a 4 drive usb caddy packed with bluray isos, it stuttered with a lot of the isos, particularly the bigger isos. I trawled through the Mederer forums and it was suggested there that something was broken after version 2 firmware with respect to BDISO's. Someone stated that going back to version 2 worked fine. Sure enough, I reverted to v2.1 (or 2.2 - cant remember) and they all work fine now....
cheers,
Neil
note didnt try the V4BETA4 right enough