Transparent interlacing (no deinterlacing) to SDTV problems

[csy]

I wish to transparently forward the PAL interlaced signal from my TV capture card (ATI 550) to the TV-out (SDTV) without having to deinterlace, because the picture quality is far superior compared to deinterlacing when viewing on a SDTV. I have had partial success because it does actually work, however I can't keep a lock between the pixel rows and the TV-out raster lines. It all starts off fine but after about a minute the pixel rows drift into the next upper or lower raster line, therefore reversing the field order, resulting in double-difference reverse interlacing which looks a complete mess. The drifting oscilates backwards and forwards between the lines very slowly, with a cycle time of between 1-5 mintes.

Has anyone tried this and got it working?

I have made sure there is no scaling or overscanning in the GBPVR config.xml, and i have proven the 720x576 image (incl the captured black areas) is precisely filling the 720x576 screen (using a monitor because you can't see the edges on the SDTV).

I also thought it might be genlock related, however my ATI 9550 video card has inbuilt genlock according to the ATI website, and I also experimented using video "flip" to reverse sync the renderer to the TV-out encoder but make no difference.

I have tried using VMR9, VMR7, and Overlay renderers but no difference.

I have tried using a Nvidia 6600GT video card, but no difference.

I have tried live-preview with no interlacing and also encoder pass-through, and time-shift, which all work but suffer from this drift problem.

I have tried multiple MPEG2 decoders (all set to weave, i.e. no deinterlacing) and no difference (Intervideo, Cyberlink, DScaler5, FFDShow, PureVideo).

Any help would be appreciated!