hi , i am using ubuntu 14.04

my server goes offline in evenings every 2 days , today it happened 3 times and it made me crazy really. it was happening 1 month ago , idk how it fixed but didnt happened for 1 month. now again started. there is no reports on all logs (kern,system etc) i also tested my ram(i thought maybe a hardware problem) but still i couldnt find my problem. can anybody help me about that ? thx a lot