Firestick sleeping too much

[Odd-no-more]

I have a problem with a firestick, when left it goes into sleep mode but then will not wake up when I want it. When I reboot it (on/off) it's fine, I've done a factory reset and it's still the same, as it is under a year old I contacted technical they want it sent back for a repair, so it's going in the post this morning and hopefully back when we get back from our trip later in the week, luckily we don't use it in the van.
Has anyone else had this problem?