And by berserk I mean something along the lines of pegging a CPU core out at 100% and not coming back... generally accomplished when switching from code to design view, or alt-tabbing back from another application to SPD when it's in design view.

1. Having a DataView on the page. Your chances of SPD going into berserk mode increase if a) you have more than one DataView, b) the DataView has information joined from multiple lists, and c) you are previewing with sample data rather than "faked" data.
2. Having script blocks with code inside them (rather than using the src attribute). It doesn't matter if they are run on the server or the client.

In instance 1 I can go a fairly long time between crashes, especially if I try to make sure I use faked data. In instance 2, if I'm working on the same page and alt-tabbing back and forth between the browser and SPD, I can crash SPD pretty much at-will in a few minutes.