Hi i have a W910i and my phone keeps on turning off every minute by it self and the battery is still charged. what can i do??

Its a software problem,give it to the shop.
To test:

• Press the On/Off key for a minimum of 10 seconds to turn the phone on.
• Sometimes, you need to remove the battery to clean dust, dirt and corrosion from the connectors on both the battery and the phone. You can clean the connectors with a soft brush, cloth or q-tip moistened with any type of unflavoured, colourless alcohol. Make sure the connectors of the battery and the phone are totally dry before you put the battery back in the phone.
• Also, check for damage to the connectors of the phone. If you have an original Sony Ericsson battery and your phone displays an 'alien battery' message, cleaning the battery as described above might help you correct the problem.
• If possible, try your phone with another battery. If the phone turns on, you might need a new battery. Before replacing your battery, make sure your charger works properly.
• If your phone has a memory card, remove the memory card and try to power the phone on. If the phone turns on, you might need a new memory card.

That is a technical problem and most fortuneiatly has to be handled at your wirless provider