I use an 8400gs in two boxes and I don't get freezing video.
do you mean 1080i video ? which deinterlaicer are you using ?
my settings are different. I didn't have to change the values for the following, they work by default:
video.output.vdpau_enable_inverse_telecine:1 video.output.vdpau_honor_progressive:0 video.output.vdpau_skip_chroma_deinterlace:0 engine.buffers.audio_num_buffers:230 engine.buffers.video_num_buffers:500