I bought a used K750 a while back. The first thing I did was that I updated it firmware using SEUS. I also tried R1CA021 as it was said to be the most stable version. Later I updated the firmware from SEUS again.
The reason was that I am facing random shutdown. Its just shutdown for no reason, like at night when I am sleeping and not using it. Some times when I start a call or if it receives a call it shutdown after first bell. Same is the case with messages, but it rarely happens. Sometimes it go without problem for week, and sometimes it keeps on shutting down when i am trying to start it. I have been switching between R1CA021 and R1BC002, but to no vail.
One thing I have noted is that it usually have this problem when the battery is like below 40%. I changed the battery but the problem persists.
One change that I have noted is that, when I was using the original (old) battery, when I start it (after a shutdown) it sometimes crashes while showing that Sony Ericsson logo. But if it proceeds it works fine for sometime. But with this new battery, I never had that problem. It now shutdown while searching for service of after it has searched for service.
I am clueless here of what might be the problem. Its doesn't seems like a software problem because it also shutdown when I am not using it.
I wanted to discuss this with you guys as there might be someone who know may have faced this problem, before I actually take it to some engineer...