Network Pinging Problem

Hello All.

I have an ISA firewall server and I am experiencing the following network problem.

- I restart the firewall server
- I logon, I "ping 192.168.10.x -t" and I get "reply from 192.168.10.x time < 1 ms"
- After 10 minutes of normal ping operation with "time < 1ms" then the problem starts occuring....
- I get:
reply from 192.168.10.x time < 12 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 12 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 12 ms
reply from 192.168.10.x time < 13 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
reply from 192.168.10.x time < 1 ms
I tried pinging to other PCs on the network and the same problem occurs.
Constantly every nine-ten replies with time < 1 ms I get one reply with time < 12 ms.
Also, notice that if I restart the ISA server then for about 10 minutes I am not experiencing the problem. After 10-15 minutes the problem occurs.

Any ideas what is causing this problem?

Thank you in advance

Stergios.