Nighthawk C7000 AC1900 Modem/Router Lag Issues

Hi there. I'm having a strange issue that Spectrum Internet will not help me troubleshoot since I am not using their modem. I'm using the C7000 as my main modem and router and experience a high amount of lag for no apparent reason. I have tried both hard-wired and over WiFi and experience this lag. Now, I've ran multiple speed tests through Speedtest.net and get perfect results (~20ms ping, 115 down, 11 up) just about every time. This occurs through a hard-wired PC and through WiFi on my cell phone using the app. The problems occur in games such as Rocket League on Xbox One and World of Warcraft on PC.

Here's the tricky part though... it's sort of random. On Rocket League, I connect to one of their servers every match. Some matches, the latency is great (<60ms) and will remain great and consistent for that match. Other matches, I'll connect and have a high latency (~150-300ms, which is constantly fluctuating throughout the 5 minute match). On WoW, I'll have a "Home Latency" in the mid-30s but a "World Latency" in the hundreds, sometimes as high as 400ms.

There doesn't seem to be any rhyme or reason for this to be occuring other than bad lines going into my house. I've made sure to run a dedicated coax cable to my modem and made sure there were no kinks in the line. I'm going to be getting Spectrum's modem just to see if that helps in any way but I'm really hoping I can fix the issue myself and continue to use my modem because it's much better than anything Spectrum can give me.

Any help would be appreciated, thanks!