Seconds Behind Master in Cloud Service
If all clocks are sync'd, I would suspect high write load on the Master. Why?

The Master is responsible for doing two things:


Executing the SQL
Recording the SQL and its timestamp


If there is any intermittency between steps 1 and 2, this could account for what seems to be replication lag.

Since there are stable servers, I suggest resetting the relay logs on the unstable servers by doing the following:


STOP SLAVE;
SHOW SLAVE STATUS\G
Get LOGFIL from Relay_Master_Log_File
Get LOGPOS from Exec_Master_Log_Pos
CHANGE MASTER TO master_log_file='LOGFIL',master_log_pos=LOGPOS;
START SLAVE;


The CHANGE MASTER TO command erases the relay logs it currently has and starts fresh.

Give it a Try, and see if this clears up the replication lag !!!