Well a spot of good news. I've just discovered that it is indeed a problem with vegas' ac3 encoding. I have just tested a problem section using Maven3D pro and the problem has gone away, but the audio is a little quieter (could be due to the wav exporting because Vegas is reporting the audio as too loud when the peaks clearly show that they are well below the threshold). I might just skip the xvid completely and go straight to the PAL & NTSC DVD-5 versions because the encoding quality of the xvid, even with a 2 disc size, is below par. But i may still do an xvid if i can get it looking better