I have an SP3 with W8.1 installed on it.
If I use it with the keyboard cover it regularly becomes un-responsive to any keys (including ctrl-alt-del) or the track-pad. [Edit: never happens with the surface dock or a toshiba US dyna-dock, I haven't tested whether I get the behaviour if plugged into mains. I've only observed it when running on battery.]
This has only started happening in the last couple of months and only happens when using certain applications - Chrome browser (Firefox is fine), excel, outlook, word.
Pressing the power button to lock the device, then logging in again fixes the problem temporarily but it recurs every few minutes.
Going into device manager and removing the drivers for the keyboard cover (they auto-reinstall) fixes the problem for a day or two, but it eventually recurs.
I've googled the problem and it seems driver deletion is the recommended solution. But it isn't a permanent solution. Can anyone suggest one? I'm hoping the fact it only happens with certain applications is a clue.