I'm having problems when watching downloaded (legal) .avi files.

Sometimes I can watch a whole download on either VLC or Media Player Classic without problems. But sometimes I get a really choppy and jerky playback??

I don't understand as sometimes it's fine, and sometimes it isn't??

When it happens, even after restarting my laptop the problem remains. I go back a few days/hours later and it's fine.

So annoying when I get to the last 30 mins of a film and the playback goes all choppy and jerky!!

Any ideas?