I own this tablet for almost a year now, at it is never stable - so I'm not very surprised of your problem.
Once I even had similar problem with restarting, which i solved by turning it off completely by holding the power button a longer time (10 sec. and more until it vibrates) and connecting it on charger for a couple of minutes, and after about - 5-10 minutes I could turn it on normally. (Btw. I had SIM and microSD card inserted)
If nothing else helps and you don't have important data on tablet, you can always try firmware reinstall... I've stopped counting my reinstalls in the last year
If you have original ROM than you can try flash it. There are two version of device S7-931u (3G model) and S7-931w (Wi-Fi model)
I have 3G model and use "SP19" ROM that can be downloaded from Huawei page.
(Beware, that is not the same ROM that you probably got if you own Croatian T-Mobile device!)