Postgresql Streaming replication - monitoring

I have setup streaming replication for my postgres servers. I am using PostgreSQL 9.2.6. Replication seems to be working fine. I am monitoring this with two ways using nagios:

1. Log_delay and
2. Byte_lag

I am very frequently getting critical alert for log_delay and in the same time byte_lag is not throwing up any alert. log_delay will be OK after 1 or 2 minutes. Can any one please suggest me If I am missing something in my setup?

Queries are given below.

Log_delay

SELECT  
    CASE WHEN pg_last_xlog_receive_location() = pg_last_xlog_replay_location()  
    THEN 0  
    ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp()) 
    END AS log_delay; 

Byte_lag

SELECT sent_offset - ( replay_offset - (sent_xlog - replay_xlog) * 255 * 16 ^ 6 ) 
       AS byte_lag 
FROM ( 
    SELECT client_addr,
           ('x' || lpad(split_part(sent_location,   '/', 1), 8, '0'))::bit(32)::bigint 
           AS sent_xlog,
           ('x' || lpad(split_part(replay_location, '/', 1), 8, '0'))::bit(32)::bigint  
           AS replay_xlog,
           ('x' || lpad(split_part(sent_location,   '/', 2), 8, '0'))::bit(32)::bigint  
           AS sent_offset,
           ('x' || lpad(split_part(replay_location, '/', 2), 8, '0'))::bit(32)::bigint 
           AS replay_offset 
FROM pg_stat_replication ) AS s;