Very, very strange problem with slow internet

PretentiousGit

Reputable
Jan 31, 2016
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I wasn't entirely sure whether to put this in the networking section or the power supply section, but here goes.

My internet has been slowed to a halt for the past 4 days or so and today I noticed a pattern where it would be fine until I turned my desktop PC on. I decided to run multiple speedtests on my phone and laptop with the computer turned on and off, and with it turned off, the speeds were totally normal (40-50mbps). As soon as it's powered up (before it even boots into Windows), it drops to 2 or less.

Here's the strange part - I ran these tests with my computer not connected to the internet in any way, shape or form. Could my power supply be failing and somehow causing problems with the modem? Is my computer just cursed? Should I bury it in the garden and offer up a blood sacrifice? Any help would be much appreciated.
 
Solution
When you say it was not connected to the internet, do you mean you unplugged the network cord to it, or just was not online in the browser?

A bad network card can cause network issues when the network is almost unusable. Test things with the computer totally unplugged from anything in the house, remove the network cable, unplug any wireless cards, etc... then test things. Then try a new network cable if you are wired. If issue is ongoing, try a different port on the router/switch. If issue still happens, buy a add-on network adapter (about $15) and try using that one instead of the motherboard.
When you say it was not connected to the internet, do you mean you unplugged the network cord to it, or just was not online in the browser?

A bad network card can cause network issues when the network is almost unusable. Test things with the computer totally unplugged from anything in the house, remove the network cable, unplug any wireless cards, etc... then test things. Then try a new network cable if you are wired. If issue is ongoing, try a different port on the router/switch. If issue still happens, buy a add-on network adapter (about $15) and try using that one instead of the motherboard.
 
Solution