Ptterb can you post your full code please?

I copied your code.
Added fvidscale_cap to pipeline, with:

self.player.add(self.source, self.scaler, self.fvidscale_cap, self.sink)
gst.element_link_many(self.source,self.scaler, self.fvidscale_cap, self.sink)

From the main program I create a new QWidget, and pass its winId() to Vid constructor.
The widget start loading, but crashes.

The output says:
should be playing
Segmentation fault