Have you tried anything yet?
Not sure what items you may have installed or how so the first and best foot forward would be to start your fone in safe mode and use it thusly for a short period to determine if its market/YOUinstalled apps or the OS that is having troubles. If in safe mode the troubles go away it will point to apps installed from Market or were ever. If it dose not go away it will be the os or worse yet hard ware.
If its a Market app Try uninstalling this.
If its the OS try updating/reinstalling via the supplied PC suit from Sony.
If that doesn't work I would be sending it to were you purchased it and get the thing looked at.
To boot into Safe Mode:
Turn off the device
Power on
During start up, hold down the menu button as soon as you feel the first vibrate
Continue to hold menu down until phone has completely booted