Random; drastic drop in download speeds

Good evening,

We have been eperiencing random, drastic drops in download sppeds over the past few months.  "Normal" speeds are usually around 250Mbps.  When these drops occur, speeds go down anywhere from 0-2mbps.  The network may stay stable for a day, a few days, sometimes up to a week, it really is random.

EQUIPMENT:

Arris SB8200  (wall outlet to splitter; one split to SB8200, one split to TiVo)

Google Nest WiFi router (ethernet cable from SB8200 to router)

SB8200 Upstream Bonded Channels:

Channel 1; power  54.0 dBmV

Channel 2; power  54.0 dBmV

Channel 3; power  52.0 dBmV

Channel 4; power  52.0 dBmV

The first two channels appear to be in spec with Arris' documentation.  Channels 3 & 4 should top out at 51.0 dBmV according to Arris.  I'm not sure what the levels should be with Xfinity, or if that even could make a difference.

Any help would be greatly appreciated.

Jeff