XPS13 Ambient temperature start problem

My XPS 13 9360 (HD, i5) fails to reboot when mildly warm - anyone else with this prob?

It was bought last Tuesday. (The second one, as the first had a display panel fault). This second one failed to boot when left on a  table by a radiator. It was barely warm to the touch. I did a hard reset (button held for 10 sec) and then it rebooted. But I lost my work in progress.

I did a check using my airing cupboard and a digital temperature probe - at around 30 degree C and repeated the problem, fails to reboot when warm (at 29.9 deg C). The upper temp limit is 35.

My Toshiba is still fine at 47.5 degC.

Any Ideas for fixing - or is it as I suspect another HW fault - battery maybe?

Second posting Update. I have removed my apps and checked the setup and it seems like any change in ambient temperature, up or down, causes the laptop not to start from sleep nor from shut down, when using batteries or mains.

Only way to wake it is with the 10second button" throat cut". 

I'm staggered that 2 out of XPS's have failed, Dell quality is poor Game over, money back time. What's a reliable brand?