Alienware x51 can't connect to wifi

Hi,

I just received my PC. When I was excited to use steam, the problems appeared.

- When I open the computer, it shows all wifi options I can connect. And I am connected to my wifi.

- When I open the steam and download games, it works for a while. The peak of downloading is 10Mb/s.

- After a few minutes, the downloading speed is stuck on 0 bytes.

- When I open the wifi connection, there is no option I can choose, and the wifi is disconnected.

- I open the device manager, the wifi adapter is disappeared.

I found some resolutions from other posts about this problem. I disabled the power saving option of the wifi. There was still nothing happened. Then I updated the driver of the wifi adapter. It worked. However,

- The speed of the wifi is still so low.

- At first, when I download games from steam, the peak reaches 10Mb/s. After a few seconds, it will drop to under 100Kb/s.

- At this time, I even could not browse website. And I think the wifi is disconnected, but the system does not show this.
- After 1 min, the speed comes back.

- And it repeats the speed low and high.

My Alienware is X51 R3. The wifi adapter should be 7265 2*2 version. My wireless router is Arris SBG6700-AC.

I don't think this is my router's or the hard drive's problem because my Macbook Pro never disconnects while downloading via steam and keeps 10Mb/s speed for downloading and writing to hardware.

Could some one help me solve this problem?

Thanks.