If people are downloading, uploading or using the network, of its just getting a weak signal it may be the cause stuttering issues. Hard wired connection is best especially when transcoding, if you can find a way to wire into your network do that otherwise ensure no one is using any p2p software, downloading anything or doing any thing network intensive at it will hinder the bandwidth available on your network for your Media Server.
As well as what are the specs on your computer that you are using? An older machine may have problems playing back.
Core2Duo E8400 (@ 3.0), 8gigs of Mushkin, Vista Home Premium SP2 64bit, 2.5 TB of Space, Wired to Linksys WRT54G wired to 80GB MGS4 PS3 on Sharp 32" 720p LCD TV