Laptop Crashes During Sleep Mode?

Over the last week, my laptop has started to occasionally crash and switch off when it's in sleep mode - after this happens, it won't turn on when the power button is pushed, it'll only work again when it's plugged in. Once it's switched on again I get the black screen that tells me it wasn't shut down properly. I've followed several of the suggestions I've seen on various threads (the power settings upon closing lid are set at 'sleep', hybrid sleep is switched off, I keep it well ventilated, the drivers and BIOS are updated) and still it happens. I haven't done anything significantly different on the laptop, no new installations or updates that I can remember, it runs perfectly fine on battery power when it's awake, nothing else at all seems wrong. I only updated drivers and BIOS etc. after the first few times this happened.

I'm sure it's significant that the laptop is usually fine in sleep mode (even on battery power) and wakes up properly if it's been sat still in one place, and it only seems to crash while I'm moving it i.e. the power light is on when I pick the laptop up, I carry it to another room, set it down and the light has gone off. Also, in one instance after moving it I successfully woke the laptop from sleep, went to plug it in, and when I looked it again it had turned off.

It's still under warranty for another 60 days so I plan to take it in to be checked (I'm not smart enough to start taking apart a laptop, much less actually understand how to fix it), but until I can get to the store next week I'd really like to know what this could be.