Happened to me a few times as well. Mostly when I was using the media player. The OS should make receiving calls the absolute priority for the CPU but it doesn't seem like it.

The media player just brings my N900 to its knees. It is incredibly slow, sometimes takes almost a minute to start playing a song and then stutters and skips for the next 5 minutes. And that's when it doesn't just crash. Plus everything else in the device becomes super sluggish. I just can't understand it.