Computer not using full bandwith

[mostaffo]

My computer has a 100.0 Mbps connection. If definately has the capacity to download at least 1-200 kbs. It will download at this speed for a few minutes. After a few minutes have passed, it will slow down to about 20kbs. It will stay at 20-30kbs for like 20 minutes and then go back up to 100kbs for a few minutes and then back down again. I have searched the forums for possible solutions. I have reinstalled limewire, deleted the .limewire folder, and allowed it on my firewall program. I am using Norton and do not see a firewall icon on limewire. I have read in some of the forums about isp's limiting or blocking p2p programs. Would anyone know if cebridge is limited or how I would go about finding out.

Thanks

Some stats:
Dell 4300s
processor: pentium 4 1.5 gig
o.s.: XP(service pack 2) home
RAM: 512 ddr
antivirus: norton
isp: cebridge cable