Improptu restarting of Macbook Pro while in Sleep Mode
This is almost always hardware. Each time this happens, when you notice it, run sudo sysdiagnose from terminal.app and keep track of the times. You might find some clues looking at the system.log in each case to narrow down what was happening before the failure.

Also, you can start the Apple service online - sometimes they offer to run a diagnostic remotely, but usually that's for iOS and not Mac, but it might not hurt to request info on what a hardware repair might look like and/or cost from Apple. See https://locate.apple.com for your options there.