sleep of death.. help please !!

Hi.. my phone is xperia T2 ultra dual (D5322) running on 5.1.1 official firmware ( not rooted)... since about 3 weeks it becomes unstable and automatically restarts whenever goes to sleep mode... but when I use it or charge it, it won't restart. this consumes the battery, and it's very annoying :(
is there any solution other than rooting or resetting it.... ?
thanks

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