Using a VPN did the trick for me. I had constant 150-200ms ping, down to 80-100ms.
I don't even connect the VPN to an US endpoint but in my own EU country.
It seems there was a peering problem or bad agreements between ISPs in the middle or the chain, while using my native ISP connexion, causing a sudden huge increase of latency after reaching the US.
Try doing a tracert
www.project1999.com, and track the latency values at each step. Then try with a VPN and compare the result.
For what it's worth, I have slightly better results (-10-20ms) on p99 with expressVPN than Nordvpn.