Internet keeps disconnecting between 11:00 AM and 5:00 PM

Bounty alert! 50$ Amazon gift card for solving my issue

Hello experts, 

My internet disconnects and connects back in 3-4 minutes many times between 11:00 AM and 5:00PM. I am having this issue for 10 months. It was at peak from July to Nov and now started again since  march. 

I have already complained may times. Xfinity technicians has visitied many times, but were not able to solve problems.

1) All cables verified by both apartment maintenance and Xfinity technician.

2) Signal strength is more than good

3) Xfinity tech installed debug modem (+ router) to track bandwitdh. Used it for more than a month, but they didn't came up with any answers

4) Tested with Xfinity's other modem also, same problem

My Equipments:

1) Nighthawk multi-Gig speed cable modem

2) Nighthawk AC3200 Router 

I have tried so many things but was not able to find the reason why it is happening. The fact, that it happens between 11:00 AM and 5:00 PM is more puzzling.

So sharing some details here, hoping it will catch someone's attention who will help me in solving  this puzzle.

Thanks