hmmm... I use vrc_buf_size=1835 as recommended for KDVD
the gop is standard. keyint=15...
Edit: Ok I figured something out... My source is the movie "Swimming Pool" PAL DVD, and it seems to me it is mastered in a very weird way. looking at it through vdub, it seems that every third frame or so looks filtered in some way.
It's like this : Noisy Frame, Noisy Frame, Plain Frame, Noisy Frame...
You get what I mean. These Plain Frames lack details, and look simply bad. Maybe the flashing and artifacts appearing in my encode are due to my script, which cannot cope with the fact that the noise suddenly stops randomly and then comes back again in the next frame...