Network problem

i have 30 computer network.
using 4 dlink switch.

when i ping specific computer belong to specific one switch their is some packet loss
means
i ping with this command
ping 192.168.1.21 -t then its ok
but when i ping with 5000 or 10000 bytes with this command
ping 192.168.1.21 -l 5000 -t
then i could not able to get reply continuously
i got request time out with every 3 or 4 reply.

what is the problem that i do not understand.
this problem start before few days.
and i have network since last one year.

what may be the problem
please guide me.
Regards
Hardik