My suggestion is to open 3 command prompts and start three separate pings (with the -t switch)
First one to the IP address of your router, something like: ping -t 192.168.137.1
Second one to your ISP, something like www.ISP.com
And a third to a reliable internet site (www.google.com)
Make sure each ping has the -t switch.
Watch them for a while to check you're getting consistent pings, there shouldn't be much variation at all as long as your link is not being used much.
If it's all good, launch a multiplayer game (maybe a custom game or something so you can ALT-Tab when your ping goes bad)...
When your game starts misbehaving, Alt-Tab and watch your pings. Generally the 'problem' will show up as long pings, highly variable pings or dropped packets on the closest ping to you. So if your router ping deteriorates, then the problem is on your home network, if either (or more likely both) the other two pings are unreliable, but your home network ping is fine, then you've isolated the issue.
It's also worth unplugging or turning off all other devices on your network (including wireless devices). If a device is hammering your internet connection your ping will suffer.
The other thing to do is monitor your network traffic in Task Manager, just in case a background task on the computer is hammering your internet connection.
If the PC is only thing connected to the router, and it's not hammering your internet connection, AND the pings demonstrate that your home network is fine while the remote pings (to ISP and/or Google) are dodgy, then you have good information to contact your ISP and start to troubleshoot on their end.