AT&T Fiber in the SF Bay Area is flipping bits
Similar problem, on the peninsula, running ATT Fiber via Sonic. 
There is a known issue with 2 part numbers of ONT that are incompatible with ATT main station, resulting in packet loss, accrued errors, etc. 
An ATT service tech can come out to replace this in 3 minutes. Have had zero problems since replacement, after getting knocked off network many times per hour.