Disk First aid unable to correct error in time machine snapshots

MBP 15" 2017 OS 11.1

Glitchy performance, occasional freezes, buggy.

I ran Disk First Aid and got this

Running First Aid on “Macintosh HD - Data” (disk1s1)

Verifying the startup volume will cause this computer to stop responding.

Verifying file system.

Volume could not be unmounted.

Using live mode.

Performing fsck_apfs -n -l -x /dev/rdisk1s1

Checking the container superblock.

Checking the EFI jumpstart record.

Checking the space manager.

Checking the space manager free queue trees.

Checking the object map.

Checking volume.

Checking the APFS volume superblock.

The volume Macintosh HD - Data was formatted by newfs_apfs (748.30.63) and last modified by apfs_kext (1677.60.23).

Checking the object map.

Checking the snapshot metadata tree.

Checking the snapshot metadata.

Checking snapshot 1 of 1 (com.apple.TimeMachine.2020-12-23-194649.local)

warning: snapshot fsroot/file key rolling tree corruptions are not repaired; they'll go away once the snapshot is deleted

error: Cross Check : Mismatch between extentref entry reference count (1) and calculated fsroot entry reference count (0) for extent (0x345258c + 13)

Checking the extent ref tree.

Checking the fsroot tree.

error: found file extent gap (id 109403882) at logical address 36864 (expected 0)

error: found file extent gap (id 109599970) at logical address 61440 (expected 0)

error: found file extent gap (id 109646643) at logical address 40960 (expected 0)

error: found file extent gap (id 109648373) at logical address 40960 (expected 0)

error: found file extent gap (id 110103713) at logical address 57344 (expected 0)

error: alloced_size (49152) of dstream (id 110568184) does not match calculated size (0)

Verifying allocated space.

Performing deferred repairs.

error: Unable to perform deferred repairs without full space verification

error: Try running fsck against the entire APFS container instead of a volume

The volume /dev/rdisk1s1 could not be verified completely.

File system check exit code is 0.

Restoring the original state found as mounted.

Operation successful.

I ran DFA on the Container Disk as suggested, no joy. In the meantime, Time Machine stopped working only on the MBP. Other networked iMac's were fine. First I deleted any third party anti-virus, nothing changed. Then I deleted other third party programs, no joy. Finally deleted the MBP T M back up, deleted the snapshots using terminal, set up T M again, back-ups started working and glitches disappeared...for about 2 days. Error is back in DFA as above, etc. I turned off Time Machine back-ups, but the snapshots remain. BTW, the actual snapshot #1 changes from day-to-day as it's supposed to, but the error remains identical for every Snapshot #1.

Any ideas?

Thanks