Random ping spikes

I’m experiencing random ping spikes (from 20ms to 1000ms and more) every 15-20 seconds that last for 1-2 seconds. This is especially noticeable during video calls and online video games making them unplayable. 

I’m using a NetGear CM1150V Cable modem (I’ve checked different WiFi access points/routers and the problem persists). I have tried pretty much everything under the sun to troubleshoot:

>Power cycled both modem and router 

>Restarted modem through Xfinity Assistant

>Tightened coax connection on modem and outside and checked all cables

>Factory reset both modem and router

>Dusted modem and router

>Changed ethernet cables connecting modem to router and router to desktop

I’ve had this issue resolved two times in the previous months after talking to support agents that were able to escalate my case and the advanced support team did something on their end after which I enjoyed a month of uninterrupted service. However after a month I started experiencing the same, repeated the sequence and got it resolved for an additional month. 
this time I’ve had a customer service experience that made me cry (literally). I thought it was impossible to design a process that infuriates even calm people. I was wrong. First I’ve spent a lot of time in the online chat where the system has disconnected me a few times and I had to re-explain my situation to a new person over and over. So I decided I call. 
Before I could even talk to a customer rep, the IVR got me on my knees. It offered support through online chat, then through text messages, then tried to reset my router remotely (and I’ve just done this with two consecutive agents in online chat) and only after 8 minutes of humiliation it connects you to a customer rep who is even less resourceful than the machine was. The absolute pinnacle of emotion was reached when after 2 hours on calls with tech support they finally agreed to get me a call back from the advanced tech support (that I already know can fix my issue in like 5 minutes). So I hang up and wait for the callback, so my phone rings, I answer, IVR says: to talk to the team press 1, I press 1 it says “incorrect option” and hangs up on me. Then it repeats. And again. In the end it says something “well, we were unable to reach you, so we’re closing your case”. 
The second time I’ve repeated the whole process, and a support agent finally agreed to connect me to an engineer, but what she did - she has just transferred me to a NetGear hotline. Another company’s hotline. That also starts with an IVR. And obviously they didn’t even know what I was calling about. 
On try 3 I tried texting and asked to talk to a manager and they have scheduled a call back for tomorrow. The manager swore it’s going to be a live person, not an IVR. I don’t believe in that. 

How can I fix this?