packet loss when pinging to gateway of router

Dear all

I had been working for company its has 4-5 switches -1 router wndr4500netgear and almost 101 pcs working with all are in lan..

We had taken leased line internet connection 15mbps and usage not exceeds more than 11mpbs for lan and average usage is 6 mbps

From Last Three week I am facing internet slow down in network when i had tested ..
pinging to router gateway.192.168.1.1 -t is working perfect with 0% packet loss
pinging to router static ip 182.xx.xx.x is working perfectly same 0%loss

when ping to gateway of static ip frequently getting packet loss what should i Need to work on...

I had connected directly to a ISP device with single laptop its Working perfect and for all gateways its fine ....

Please can you friends assist me so that i can sort-out this issue I will grateful thanks to all ..

Thanks in advance ..