Having the exact same problem.
I've been monitoring my connection for about 2 months for T3/T4 errors and documenting when they happen. Xfinity placed a 30day watch on my connection to record the T3/T4 errors on the ISP side.
Up until recently, the logs between my modem and Xfinity matched. However, as of 4 days ago the logs on my SB8200 modem, have disappeared and seemed to have stopped recording. Not sure where they went.
Any insight from Xfinity would be great. Intermittent connections have been a regular occurance.