Less Download + Uploading Kills Everything

[KuraiRyuu]

First: My internet is supposed to be around 200 down and 13 up, but when testing on mine and my sister's computers it comes up with about 20-30 down and 10 up. My phone and another computer in the house shows having the full internet it should have.
Second: Whenever I try to upload something, for example a 400mb zip file, my ping to goes up beyond 1000-5000 and my upload/s goes down below 1mb until the internet barely works whatsoever, and the upload sometimes fails.

Router/Modem Type: XB3
Router/Modem Model: (Cisco) DPC3941T
Motherboard: ASRock Z97 Extreme4
Power Supply: EVGA Supernova 750 G2
Wifi Adapter: Cisco AM10 Valet Connector USB
OS: Windows 7 64bit (Home Premium, Service Pack 1)
ISP: Xfinity Comcast.
Connection Type: Don't know.
Devices connected: 7

 

[Kewlx25]

How are you connected to your modem?

 

[KuraiRyuu]

How are you connected to your modem?

I'm just connected to it by the AM10 wireless adapter, unless you mean connected some other way.

 

[KuraiRyuu]

I've found that in the status for the internet connection, it shows the speed as 144, then immediately goes to around 20 when I try to download anything, and about 6 and probably lower when I try to upload.
I've also tried with and without all my firewall stuff(Windows, Avast, Malwarebytes). And I've scanned and found nothing with Malwarebytes.

 

[Kewlx25]

Good chance it a wifi issue and not an "internet" issue.