mobile9 -

make sure you have a goog battery and then try to factory rest your phone if didn't work out good then you have to update your phone

Sorry i meant Good not goog

i think its a virus. connect your phone with pc and scan both memories if it will not solve the purpose then format both memories (phone and card)

Try removing your memory card
then switch on your phone

If it works then replace your memory card

If you still face the problem then
Try to factory reset your phone (In the settings menu)

Well, this seems to be common problem wuth this phone. I got it too, and without any reaseon it restarts every now and then. Try factory reset, but I think it won't help. It didn't work for me. So it may be just the way it is. I think it's 索尼爱立信's fault...sorry but there is not clear solution for this problem.