how to make Linux box working well as router

[clefstar]

I have configure my Linux box as a router which is serving as a gateway
for the clients on my LAN but sometimes it keeps dropping down which cause the client not reaching my ISPs gateway router and any other extenal server whiles the Linux box itself is reaching everything extenal. When I restart the Linux box then everything will be working for sometime and godown again.

What could be the problem and what I do to solve this problem.

Please anybody with a solution should please help me.

Thank You.