Dell XPS L502X

Hi there, 

My XPS laptop has recently started playing up (full model: XPS L502x : 2nd generation Intel Core i5-2410M processor 2.30 GHz with Turbo Boost 2.0 up to 2.90 GHz). 

Instead of sleeping when I leave it, it turns itself off, so when I turn it on again I get the "Your computer wasn't switched off properly etc etc" screen. I haven't changed any settings (not that I know of anyway) and have run diagnostics checks, but can't stop it happening. 

It doesn't happen every time I leave it idle, it sometimes survives a whole day without doing it. And it doesn't seem to be overheating.

If anyone has an answer that would be great.

Thanks so much for your help!