intermittent extremely high ping times

For several months I've been getting occasional ping times of up to 10000ms or more. Yes that is 10 seconds!! Makes it impossible for us to work from home. It occurs a few times per hour on average  from 9am to about 6pm, not at night. Each occurrence lasts for a  minute up to 10 minutes or more. I have tried rebooting and factory reset on my old modem many times, swapping in a brand new 8x4 Docsys 3.0 Motorola MB7220 modem, and connecting the modem directly to the cable coming into the house. None of that made any difference. My downstream power varies from 9 dBmV during the day to 14 at night, with SNR of about 40 dB. Upstream power is about 34 dB. Called Comcast agents, but got no help other than to reboot the modem yet again. They are not able to send a technician to the house during the pandemic. Any advice? My wife and I will not be able to work without internet service.