Mysql Percona 5.6 slave lags gradually

I have got into slave lag gradually say every minute it lags around 5secs on slave.

So it started morning 10AM the Seconds_behind_master was 13. Now at 4:30 PM it has reached to 560.

No variable changed. Not much selects are running in there, its all regular ones. In fact we have stopped few applications thinking those could cause slowness due to selects.

This seems to be strange for me. This had never happened before.

I have below variables already set to minimize IO load on disk.

> innodb_flush_log_at_trx_commit=0   
> innodb_flush_method = O_DIRECT
> sync_binlog=0

I also have 2 more slave nodes. They are performing real good. And they are used heavily by selects on app end.

Any thoughts or any one have run into similar issue before?