Try to Disable / Enable Hardware Acceleration in the browser you're using.
Thanks but I've already tried that.
What I did notice the other week is that the stutter appears to tie up with spikes in ethernet activity so it's probably nothing to do with the GPU. I've changed the motherboard when I updated the GPU so this may be the cause.
I may be a step closer but I still have no idea of the solution. I guess I'll start with mobo drivers.
Thanks for your input though.