I'm seeing underruns. Unfortunately, searches on what causes this exactly tend to turn up people's show controller commands that have 0 errors.
M1T-T3 pa: show controller:
PAS unit 0, subunit 0, f/w version 3-93, rev ID 0x2800001, version 3
idb = 0x20045C64, ds = 0x633C17D0, ssb=0x20046E90
Clock mux=0x30, ucmd_ctrl=0x0, port_status=0x1
Serial config=0x8, line config=0x1B0202
maxdgram=4500, bufpool=128Kb, 256 particles
rxLOS inactive, rxLOF inactive, rxAIS inactive
txAIS inactive, rxRAI inactive, txRAI inactive
line state: up
cable type : T3 cable, received clockrate 44201430
base0 registers=0x3C800000, base1 registers=0x3C802000
rx_base0 registers=0x3C820000, rx_base1 registers=0x3C822000
mxt_ds=0x20783D1C, rx ring entries=124, tx ring entries=254
statring=0xC329A00, statr shadow=0x2004725C, stat_head=199
plus_pa: rx_statring=0xC329E40, rx_statr shadow=0x5006FBFC, rx_stat_head=103
rxring=0xC3297C0, rxr shadow=0x633C2CE4, rx_head=3
txring=0xC32A380, txr shadow=0x633C3310, tx_head=166, tx_tail=169, tx_count=3
throttled=0, enabled=0
halted=0, last halt reason=0
Microcode fatal errors=0
rx_no_eop_err=0, rx_no_stp_err=0, rx_no_eop_stp_err=0
rx_no_buf=0, rx_soft_overrun_err=0, dump_err= 0, bogus=0, mxt_flags=0x2C
tx_underrun_err=6327187, tx_soft_underrun_err=0, tx_limited=0(256)
tx_fullring=5147, tx_started=1582336790, mxt_flush_count=0
rx_int_count=-665103885, tx_int_count=-1870512746
Framing is c-bit, Clock Source is Line
Bandwidth limit is 44210, DSU mode 0, Cable length is 10
rx FEBE since last clear counter 0, since reset 99
Data in current interval (286 seconds elapsed):
0 Line Code Violations, 0 P-bit Coding Violation
0 C-bit Coding Violation
0 P-bit Err Secs, 0 P-bit Sev Err Secs
0 Sev Err Framing Secs, 0 Unavailable Secs
0 Line Errored Secs, 0 C-bit Errored Secs, 0 C-bit Sev Err Secs
Data in Interval 1:
