Router dropped speeds with Ethernet connection all of sudden

You could have a few different issues here and yes, the router maybe failing as well. First make sure your Router is not being hijacked and only you (who you authorized) are using its connections. I would also make sure the frequencies used by the WiFi is not being sat on by others nearby.

Can you tell us what the Internet LED is telling you?

Or if you have access to a passive Hub (must not be a switch type of hub) plug it in between the Internet port and your Internet router (daisy chained). Then plug your PC into one of the free ports on the passive hub so you can run a sniffer app like WireShark (Ethereal) Here's the download: WireShark. This will allow you to see whats running across your net.

Things I would do to make sure my router was not compromised is make sure I have the latest firmware directly from Netgear I would also alter my admin user & password so it's not at the default shipped and different enough no one will be able to gain access (complex). Make sure you do write it down so you don't forget it! Don't forget to enable WPA/WPA2 for your WiFi connections and turn off your open port so people don't drive by to gain access to your internet connection. I also recommend altering your AP names so each band is unique so the 2.4 GHz band would be ACME_2.4 and the 5.0 GHz band would be ACME_5.0 That way you know which band your system connected to. I also recommend you turn off advertising (stealth mode).

To make sure the WiFi AP's your router is using are not in conflict with other nearby AP's you'll need to download a WiFi sniffer app onto your laptop. There are a few that offer what you need as an example: CommView for WiFi which shows you graphically the different AP's and their bands.