Loss of download speed using router...help!

My Download speed through "wired" router avg. 350 kbp/s, without router avg. 1800 kbp/s. (upload is always around 500 kbp/s) ISP connection is 3 mg, only one puter and voip phone behind router. Have tried the following tweaks: (1. upgrade firmware: best results with hyperwrt 2.0. (2. have detected isp MTU to 1360..set router and nic to 1360 with sporatic better results but just barely. (3. have disconnected voip line, unplugged cordless base, reset modem and router and no change. (4. Cable feed to modem is on a DC adapter at NODE ensuring best connection to ISP line. (5. Purchased a new router and tried that but download speed loss over direct connect to modem still there! (6. basically have tried all the tweaks I can find on the 'net and exhausted "extensive" linksys helpdesk suggestions!
It's given me major headache! I should not be losing more than 2/3 download speed when I am connected to a router!
router: linksys wrt54g
modem: SurfBoard 4200
Windows xp
1 gb ram

Your help would be greatly appreciated!

 

Forgot to add that I have saved emails with full troubleshooting steps that I took with linksys techs if that would help.

 

Hello Gang...well, I have solved the slow download speed when connected to a router problem. No matter who I talked to or what forum I was involved in or where I went to research on the net I didn't find the @$%#@ help I needed!!! No one had even suggested that it could be the modem but I called my cable company and had a tech come out to troubleshoot it, check the provisioning, etc. And, guess what? YES, the problem all along was the modem. Plugged in a new one and voila, download speeds approx. 2950 kbp/s every time! So, to anyone and everyone who is having similar problems with their router choking download speeds by 2/3 or more check the modem!!! I am now happy as stink and surfing like a california beach bum...