Captured video frames are poor quality

[anno domini]

Hi All,

Wonder has anyone any suggestions.
I am capturing screenshots from an easy-cap usb device connected to a PAL
tv box.
Some of the captured images are perfect but frequently they are poor
quality.

I am seeing this even if the actual video content is static (for example
when in TV menu system where there is no movement).
When i say poor quality i mean i occasionally get what looks to be skewed
images or images that appear to be the same frame overlaid on top of itself
but off centre etc.
Sometimes I'll get a block of green over part of the image.

I'd say about 70% of the images are perfect but around 30% exhibit the
strange artifacts.

Can anyone suggest anything that this might be and/or any possible ways of
improving things?
Here are the pipelines i have been using:

gst-launch v4l2src device=/dev/video1 norm=PAL ! ffmpegcolorspace ! pngenc
snapshot=true ! filesink location=screenshot.png; shotwell screenshot.png

I've also tried:

gst-launch v4l2src device=/dev/video1 norm=PAL num-buffers=1 !
ffmpegcolorspace ! jpegenc ! filesink location=screenshot.jpeg

Thanks!
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