Makes sense, since you are working with MPEGs.
Go under Video -> Framerate -> Change to _____ frames per second and try it with 25fps. Hopefully this will make a difference. If not, then the artifacts are probably hard-coded into the video (i.e. bad NTSC to PAL conversion in the mastering process).
"if you used DVD decrypter go into tool, settings, hit the IFO mode tab and set file splitting to 'none', now when you demux it will result in one file with the whole movie."
Make sure you are running windows 2000/XP with NTFS. Win 95/98 can't do file sizes over 2Gb.