pixelization

[James]

I have a program that does stuff and displays the video.
The gst part is very simple

pipeline = gst_parse_launch ("v4l2src name=source ! "
                    "video/x-h264,width=1920,height=1080,framerate=30/1 ! "
                    "queue max-size-buffers=0 max-size-time=0 max-size-bytes=0 min-threshold-time=100000000 ! "
                    "h264parse ! "
                    "avdec_h264 ! "
                    "xvimagesink name=sink1 sync=false ", NULL); 

On different hardware I get different results, ryson with amd graphics always pixelates for a fraction of a second at startup, nuc with intel graphics occasionally pixelates at startup.

The source is a fifo. I disgard P-frames until starting with an I-frame.
The artifacts are always the bottom of the display.
The image is 1080P25 (that 30 needs attention)
I've tried every thing I can think of. Please anybody suggestions on finding the cause.
James