Replication Lag

hey all...

i have mysql 4.1.14 running in a master slave setup.. in a local
network..
The problem i am having, that the replication status on slave show
random bursts of lag.. like , in two consecutive runs of SHOW SLAVE
STATUS, i get Seconds_Behind_Master: 28984, and Seconds_Behind_Master:
0
any sussgestions as to what may be causing this will be greatly
appreciated...