SpeedStream 5660 - any NATP problems?

I have a simple 10Base2 LAN on network 192.168.1.0. The local hosts
use 192.168.1.1 as their gateway. The gateway machine has a second
NIC 10.0.0.2 connected via 10BaseT to the SpeedStream 5660, 10.0.0.1.
The 5660 is a DSL modem/router.

The gateway does IP forwarding with masquerade, effectively NAPT. The
5660 also does NAPT.

This configuration works, some of the time. Works means that the
local hosts can do their HTTP thing. Does not work means *only* the
gateway machine can do DNS/HTTP stuff. However, the local hosts can
still ping through the router and telnet to it.

For no reason, the local hosts will loose WAN access and then sometime
later (hours), with no tweaking of the 5660 in between, will regain
access. The only thing I can think of is something associated with
the double mapping; viz.,

local 5660
192.168.1.x -> 10.0.0.1 -> WAN
NAPT NAPT

The gateway machine is *not* mapping its address/port, only the local
hosts'. I have never experienced any problem with single level
mapping.

Am I on the right track here - is it some kind of NAPT problem?

Thanks.