I've almost got My WinTV card tow work but ...

I've almost got my bttv WinTV card working (I get pictures out at 25fps for
384x288 and about 15fps at 768x576 including rendering using openGL). The last
problem I have is that sometimes the buffer I read seems to either 1) have
not finnished updating before I read it or 2) Have started updating the next 
frame before I've read the whole image. See:

http://www.comp.leeds.ac.uk/drm/problem.gif

I'm trying to read/write from/to 2 frames the reading code is (VERSION 1):

the_read_function()
{
    ioctl(fd,VIDIOCMCAPTURE,&vmmap) ; // Request capture to current buffer
    src = buff + mbuf.offsets[currentBuffer] ; // Set read pointer

    if(currentBuffer==0) currentBuffer = 1 ;
    else                 currentBuffer = 0 ;

    vmmap.frame = currentBuffer ;
    ioctl(fd,VIDIOCSYNC,&vmmap) ; // Release next buffer for writing

    ..... Read out data from src (i.e. frame just written to)
}

Alternatively (VERSION 2):

the_read_function_v2()
{
    ioctl(fd,VIDIOCSYNC,&vmmap) ; // Release current buffer for writing
    ioctl(fd,VIDIOCMCAPTURE,&vmmap) ; // Request capture to current buffer
    src = buff + mbuf.offsets[currentBuffer] ; // Set read pointer

    if(currentBuffer==0) currentBuffer = 1 ;
    else                 currentBuffer = 0 ;

    vmmap.frame = currentBuffer ;

    ..... Read out data from src (i.e. frame just written to)
}


Both these combinations produce the problem reasonably frequently. Am I doing
something wrong or is it a feature of the hardware?

Many thanks

Derek Magee