I've moved a linuxbox from being my gateway (nat, etc) to behind a new gateway (a DD-wrt router). For transparency purposes, I'd like to continue using the old box services, and just have the gateway in front of it.

Here's what I did:

oldbox 192.168.1.5 LAN side
newgateway 192.168.1.2 LAN side, DHCP ISP WAN side
oldbox 192.168.0.3 former WAN side, now routed to 192.168.1.2

The newgateway is setup for class C, 192.168.0.0

This configuration works, but sometimes it doesn't and I haven't figured out why yet.

At sometimes, clients on the LAN get throughput rates with very little speed loss (using speakeasy speed test). They will run at 33M on the LAN, running through both the newgateway, and the oldbox, and get about 35M with directly connected to the cable modem.

Other times, it seems nothing can talk with anyone, and I am not sure why.

I know what I have done is a little unconventional, but it's a transitional thing, and I am not sure it is the cause of the problem, although it was the last things changed.

Ideas? And ideas on how to isolate my problem? Thanks.

On your new router, change to a 10.1.10.X address and turn off the DHCP. Point your old your router to the new one. Having two routers in the 192 class might have caused a problem.
You didnt say if your client computer was set for auto dhcp. do a network refresh on the clint computer and check the address and gateway.

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I'll give that a try, but I thought that simply going to a different class C address would be sufficient isolation.

I turned off the DHCP server, and the client, on the old box, with the newgateway was implemented.

Downstream, I figure I'll set up VLANs for VOIP, etc. so that I can segregate traffic. I'll do shaping then.

DD-WRT is a nice router, but a full blown linux box isn't bad for services, and I have a 52 port switch on the back of it.