SSH/SFTP Buffering - Fine on SCP! What?

Bit of a strange one this...

I've mapped my NAS using SSH/SFTP. When trying to play a large movie (10GB) I would get constant buffering - ever few minutes it would pause, buffer, play, repeat.

I thought this might be a speed problem with my LAN. So, I used SCP to copy the file from the NAS to the XBMC box. Total transfer time was 20 minutes which, for a two hour movie, indicates that the problem isn't with the speed of my network.

Indeed, I was able to watch the file from local storage while it was streaming over SCP.

Here are my questions...
1) What causes this buffering?
2) Obviously it's not network speed - can I increase the cache somewhere?
3) Failing that, is there a way to automatically copy the whole file to a temp drive while I'm watching it?

Thanks

Terence
XBMC 11 (OpenElec) running on an ASRock 330HT.