some IP addresses stop responding on the same Server!!!!

Hi,
I am facing an odd issue! I have got about 10 IP addresses (192.168.22.10 to
19)on my NIC and there is a one-to-one NAT configured on my firewall.
sometimes one or two of the ip addresses stop responding while others
function well. when this happens I can not ping those local IP addresses from
firewall while other addresses on the same server reply. an other issue when
this happesn is that from the server side the default gateway can not be
pinged.
when this happens a quick fix is to clear arp on the server!!! before and
after clearing the arp the MAC of default gateway is the same so I don't
think there is a problem with wrong arp entries.

any Idea about this?

Regards,
Saeed Pazoki