I haven't had reports of random crashes in many months now. That doesn't mean that they're impossible, just that if they're possible, they're very rare.
The thing that confuses me the most is that you say it can crash while you're doing something else on your computer. This makes me think this is not a Sigil issue since Sigil doesn't do anything "in the background" when you're not interacting with it.
Are any other programs randomly crashing on your computer? If so, you could have some bad RAM on your machine.