No. 2 has one of these and it has generally been pretty good.
However, a year or so ago it did start dicking around similarly to yours, though it would shutdown when in use or at rest.
It was the battery. These are not supposed to be replaceable, although it is a fairly trivial job to do so. About the difficulty of an Airfix spitfire but quicker.
As rtj says, a reset may help but it is unlikely. If it causes you no issue with loss of configuration, programs, data, then give it a go.
When a device starts misunderstanding its battery capacity it also displays the 'incorrect' statistics. So, by watching that it charges to 100%, that it discharges to 1% or 2%, the charging time etc. etc. you should be able to see if the device is misunderstanding its battery, this can happen although is less likely these days.
Check for any swelling on the back of the device. Failing batteries quite often swell and since this is a tight fit, then its frequently obvious.
It is quite possible that the Tablet is doing something. You'd notice fast discharging because the battery will get warm while it is doing it. So when the circumstances are right for it to do its switching off trick, keep an eye to see if its warm.
Shake it to see if you can encourage it to turn off. In case the battery is loose or the connectors unreliable.
Ultimately though one of these is now £40 odd quid secondhand so its not worth too much repairing. (at least, that's what they are here I cannot think its too much different)
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Tablet Turning Off 
Tablet Turning Off 