Thanks for the suggestion next010.
HGF, that is weird. As it does it from memory sticks as well, I'm wondering if it's a setting I've missed. The videos from the iPhone are lower picture quality than the memory sticks from the camera.
It shouldn't be the router if the memory sticks are doing it.
I'll try recording a low resolution video with the memory sticks and see if it's smooth with that.
{edit}I tried all three resolutions and they played fine, even the high quality, through the card reader. I then remembered I'd had a port expander plugged in earlier. Tried it with that there, problem returns. It's the grubby port expander. The expander works but it may be USB 1. It also has four, entirely pointless, high brightness blue LED's on it. I know the samsungs USB ports are not able to independently power devices like drives. So it may also be those LED's helping drag the port down. Zappo not playing smoothly may be due to the BT home hub, which has caused endless problems since we signed up for BT (now going to cancel the contract and change).