If I read your post correctly, your setup looks something like this (pardon the shortened IP addresses for simplicity).
192.168.1.0 +——-Linux Box——-+
+————+Eth1 Eth0+—+ FW
+1.254 111.160+ +111.1
If this diagram is correct, the default gateway for the Linux box needs to point to 111.1 (FW) or it will just go in circles until the TTL expires.
If I’ve not understood your description correctly, then please repost and correct the picture.