TV-Out with TNT: problems

I think I need some help from a TV-Out expert. I'm trying to build a
cheap-ass DVD viewing system from a P2 400 with Windows XP and a Riva TNT.

The TNT has VGA, S-Video and Composite output.

The problem is that I get a nice picture until XP initializes the display
driver, then I just get interference OR sometimes I get something that looks
like parts of the picture, but just in b/w and very garbled (stretched and
overlayed on itself several times).

I'm remote-controlling the system through WinVNC so I can do stuff even if I
don't see anything on the TV.

It's not the Composite / S-VHS problem (where picture gets b/w but is
otherwise OK). It's not the cables (tried several). It's not the drivers
(tried 12.40 and 41.09 and 44. something). I've tried every setting in
nVidia drivers and TV tool but couldn't get a picture, just the garbled
"shadow" of a picture.

I'm starting to believe that for some reason the TNT card outputs in 60 Hz
although my TV only supports 50 Hz (PAL-B i think).

Any suggestions? Buying new stuff is not an option.

/M