Could be UBR or could be ur system.
Try these tweeks....
Download TCP Optimizer and slide the slider to 20mb right at the end, then click optimal settings in lower right and then OK, this will auto set to what the ethernet should be....
TCP Optimizer is available here....
http://www.speedguide.net/downloads.php
Also u can download Cyber Tweek from here....
http://www.cybertweak.co.uk/
Run the Broadband Optimzer and also enable the Ethernet Processor.
Try these first and see if it makes any difference.
Also if u try these and no difference is made and u have a router, try either upgrading the firmware or unplugging the router.
These will eliminate possible probs ur end and what could be a UBR Problem.