Limited Connection - Default Gateway not Available

I have done a Google search on this. Problem is, nothing I have tried so far has worked permanently. I am not even sure if the problem is my computer or my network. So here it is:

I have an Acer Aspire laptop with Windows 8.1. My main modem is located downstairs in my house, with a laptop connected to it (it works fine, never any issues). Before, I had a router wired upstairs to connect my laptop to. Here's the problem - it doesn't matter if I am using a wireless or a wired connection, I get a limited connection. I will have Windows troubleshoot the problem, and it always says that the problem was that the Default Gateway is not Available, and it will say that it is fixed. However, this randomly happens over and over anywhere from 2 minutes to 20 minutes after I just "fixed" it. I have just recently tried running a wire across my house to plug directly into the modem. This worked for a while, but the same thing started happening. Keep in mind, there is also a laptop wired directly to the modem downstairs in my house. I believe (but I haven't checked this in a while) that if I connect to the WiFi from my modem downstairs, it works fine. Another twist - I'm pretty sure I had this same issue occur on the old laptop that I had. So I am just flat-out confused.

I have heard that this is a Windows 8.1 issue, and I have heard that this could be an ISP issue (I have Comcast, if it matters). I just have no idea what the problem is or how to fix it. I have been trying so many things, and I am willing to see if there is anything else I can do. If there is any other information I should post here to give you more information to be able to help me, I do not mind posting it at all. Just let me know, I would love for this to be fixed.

Any help is greatly appreciated. Thanks!

If it is always the same computer. Bad Network driver or third party Internet security software. If it is happening to a group of machines. Then it is a router issue.

broe23 said:

If it is always the same computer. Bad Network driver or third party Internet security software. If it is happening to a group of machines. Then it is a router issue.

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Is there any way to truly test which of these it is? Because, from past experiences, it sounds like my problem could be either of these. If I remember correctly, this was happening on my old laptop as well. However, if I connect wireless to my modem, it works fine. The only thing I do know is that it isn't a problem with the Ethernet cable, as I have tried two different ones.

andrewpr52, it is basically all trial and error.