Nokia :: N97 Mini - Locking Up - Out Of Memory

I am currently experiencing an intermittent fault with my wife's N97 mini phones, where, unexplainable, it "locks up" and prevents any functions other than the Home screen. It eventually indicates that it has run out of memory and to close applications. There are no applications running and the only way to clear this fault is to remove the battery temporarily, and then replace it and turn the phone back on. All is good until the next "lock-up", usually within 1 - 2 weeks. We were advised to update the software, but this has not made any difference. My own N97 mini has not had this fault occur to date.