Default LOW RAM problem , Apps keeps restarting , phone lags

ever since i got my new xperia z1 , i keep getting this problem:

-> after using the phone for 5 hours i realize the phone lags and pressing "back button" doesnt go back , so i have to use the home button to go to home , then i checked settings and went to Apps > Running , i see i only have like 100ram left en all the apps says " RESTARTING " and it stays that way, doesnt change [CHECK SCREENSHOT].

Only way to fix this is by Restarting the phone , then after turning it on , everything seems to be normal [800mb ram left] [apps counting time] , BUT AFTER using for like a few HOURS the same THING HAPPENDS again , and it keeps happening.

So ends up restarting the phone for like 5 times a day , this isnt normal.

Is there anyway to fix this ? or is it a bug? pls help/reply

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