OK. What are you normally doing when it quits unexpectedly? Are you editing clips, entering titles, doing effects, rendering, etc? Is there any pattern to it at all?
And By the way, to answer your real question: FCE HD does not crash as frequently as iMovie HD for me, very rarely does either one crash though so I'm not sure that's a fair statement.
Save early, save often.
Unless you're a serious hobbiest or higher, I wouldn't go to Final Cut Express just to get a more reliable environment though. The learning curve is steeeeep. Don't get me wrong, if you're into it, it is a blast, but just be aware of the learning curve. For example what you may be doing in "10 minutes" in iMovie, might take you 4-6 hours to do in Final Cut when doing it the first time, just from all the basic interface knowledge that is necessary.