Hi, This has been happening to me recently. I purchased my Desire S with OS 2.3.3 in April. I had no issues till a month back. All of a sudden my phone started switching off by itself. I come realise only when I pick up the phone to make a call and its dead. It become so bad, it was going off about 7/8 times a day. After a lot of searching and trawling all the forums, help lines etc, I came to the conclusion that it must be the battery or the SIM. I also remember that just before this issue started there was some update by the phone on the software. This could have also started the problem. But my suspicion was on the battery or the SIM since I also remeber dropping my phone 3/4 times ( hard enough for black matt finish to expose the aluminium). A week back as per the solutions, I inserted a small piece of paper between the SIM and the case. I also put another piece between the battery and the case ( care taken not to touch the contacts). It worked. Till day before yesterday, it switched off only once. Then I dropped the phone again.The phone started acting up 4/5 times. Removed the SIM and reinserted it ( the paper went back inside), removed the battery , folded the paper once and reinserted. Till now it has only gone off once. It seems to be a contact issue either from the SIM or the battery. Once you drop or knock the phone hard enough, it becomes sensitive.