The WRT54Gs are lacking in CPU power so if you've got lots of features enabled then it will affect the throughput severely. I used a WRT54GS temporarily for my FTTC connection and turning off everything including the firewall increased the throughput. Depending on the model versions I believe the WRT54GS is faster than your WRT54G but I was able to get ~24Mbps through it.

Turn off as much as you can and see if it helps?

What is the WRT54G doing anyway? Do you have a router for the FiOS or is the WRT54G doing PPPoE? NAT?

I replaced it with a Mikrotik 450G which is doing PPPoE, routed IPv4, IPv4 NAT, IPv6 and SNMP. It doesn't go over 20% CPU usage ever.