Linksys WRT54G and static route

Hi,

I just bought a new wireless Linksys WRT54G router with the following setup:

Product : WRT54G - Wireless-G Broadband Router v2
This is my network layout:
Bell DSL modem 3MB (SpeedStream) --> WRT54G (DHCP enable) --> switch--> :
1. several computers
2. Lucent Pipeline 75 ISDN router (10.24.1.10/10.16.1.8)--> isdn
line/other isdn router in the other end (10.24.1.11)
3. VOIP device

Now in the Static routing, the setup is using Gateway with setting:
- destination LAN:10.24.1.0/24
- default gateway: 10.16.1.8
- Interface: Lan&wireless

Routing table:
x.x.x.x 255.255.255.255 0.0.0.0 WAN
10.16.1.0 255.255.255.0 0.0.0.0 LAN&Wireless
10.24.1.0 255.255.255.0 10.16.1.8 LAN&Wireless
0.0.0.0 0.0.0.0 x.x.x.x WAN

The problem is if I reboot the DSL modem, WRT54G and the rest, the
internet connection from user computer works normal 3 MB.
But after a while, it becomes around 100 KBps ONLY.
Before, we use Linksys wired router with similar configuration like this
but there is no problem.
What's wrong?

For sure not in the DSL modem because if i connect directly to DSL modem
without router works welland also when the connection so slow and I call
Bell tech sup, the test remotely the DSL modem have 3 MB bandwidth.
So somehow the router could be the source of the problem?

Do I have to use Router in the Static routing? but i tried with the same
setting but users cannot connect to internet, just able to connect to
10.24.1.x network.
Should I add some more static routing (example greatly appreciate it)???

Could I use dynamic routing without problem with 2 kind of routers above?
Could somebody help please?

Thanks.
Regards,
Handy