Dell venue 8 pro wifi problem

[Gaziki]

I noticed weird problem with wifi, i did that accidently. Well, dont know why when i enter speedtest speed jumps to full, then starts lowering up or sometimes it stays 10 times lower than my actuall speed. Its when iam on another room. When i go back to room where i have router ping and speeds are perfect. And when i go back to revious room and do tests again it remains perfectly fine untill next day. WTH is going on? i have linksys wag 200g