I think I know what your problem is. I had the same kind of problems on my last (hopefully for ever) PC. Virus protection. You are using an HP media server. I believe those use some kind of windows os and some kind of virus software. When I first bought that last PC I tried watching videos. Every few seconds or so it would skip. It drove me nuts. I brought it back to the store and convinced the store manager to exchange it. I had the same problem. What caused it? The Anti-virus software kept checking for viruses. every few seconds. I uninstalled it and the problem went away. I'll bet your HP mediaserver is checking for viruses and messing up the video which your mac is reading from it's hard drive.
Try disabling the anti-virus and, if you haven't already, upgrade the ram on the mini to at least 2GB.
Try disabling the anti-virus and, if you haven't already, upgrade the ram on the mini to at least 2GB.