Bosch Classixx 1200 fails to drain/spin?

Washing machine has failed to drain a few times now. each time I manually drain, remove the sump and find no blockages and reassemble it and it works fine for a while. Most recently, my wife reported a burning smell when it wouldn't work so I removed the sump, removed and dismantled the drain pump checked all the drain pipework. Everything seems OK and it started working again when I put it back together. No problems found with plumbing or pump. I'm baffled. Any idea what might be causing this?

Richard C
January 2011