Skipping durng movie playback, driving me mad please help

I'm having problems with playback of Divx and Xvid movies. At random intervals the movie either skips forwards or skips backwards what I would guess must be a few frames(affecting the picture and audio). The files aren't currupt because the skipping can occur at different points in the same file depending seemingly at random

At first I thought it might be a problem with my disk, but the same thing happens streaming movies over the network.

I thought it could be graphics drivers but the problem happens on both my displays.

I also thought it could be a codec issue, but it happens with both divx and Xvid. All the files do seem to have MP3 audio, but I don't know how to change the audio codec. Since building this system I have also had problems with stuttering MP3 playback, but this was cured by a winamp update

I've been trying to figure out what I else I can try to fix it or track down what's causing the problem, any suggestions will be greatfully recieved

Hardware
Asus A7v8X-X (onboard sound disabled - onboard LAN in use)
XP T-bread b 1700+ @ 2.15ghz (13x166)
1x512MB Twinmos PC2700 DDR
Ati Radeon 64MB DDR [oem] using CATALYST 3.4s
VooDoo 3 3000 pc using windows display driver
Soundblaster 4.1 digital (using latest driver)
EIO RAID pc133 Controller (used to provide 2 extra IDE Channels)
1xIBM Deskstar 120GB 7200rpm 8meg buffer ATA 100 Raid card channel 0
1xIBM Deskstar 40GB 7200rpm ATA 100 Raid card channel 1
1 x samsung 16x DVD-Rom (onboard IDE pri Master)
1 x LG 52x24x52 CD-RW (onboard IDE sec Master)
Hauppage Wintv PCI card
quiet pc 300wat psu

Software
WinXp Pro SP1
Playing the movies in WMP 9