mongo master slave replication

I have set up a master slave replication in mongo.it will work fine for around 8 months.but sudeenly slave show too far behind master.then i Have increase the oplogsize in master upto 60 GB . and resync the slave.slave starts resyncing but after some time it will again start resync and this process is going so frequently.and slave never sync with master.i have checked the log file in slave and it show

Thu Mar 13 22:15:49.698 [replslave] Socket say send() errno:110 Connection timed out 10.209.135.54:27017
Thu Mar 13 22:15:49.805 [replslave] repl: DBException socket exception [SEND_ERROR] for 10.209.135.54:27017
Thu Mar 13 22:15:49.811 [replslave] Socket flush send() errno:110 Connection timed out 10.209.135.54:27017
Thu Mar 13 22:15:49.811 [replslave] caught exception (socket exception [SEND_ERROR] for 10.209.135.54:27017) in destructor (~PiggyBackData)
Thu Mar 13 22:15:49.838 [replslave] repl: sleep 2 sec before next pass