Cisco PIX question

Alright guys,
I am running into an issue that boggles my mind on a brand new Cisco 525.
For the sake of troubleshooting i have moved the config to a lab environment before i move it into production for the company.
I have 2 computers plugged into a gig-switch, that switch is plugged into the PIX. The PIX is plugged into a 54Mb Microwave line (54Mb up and down).
Here is the issue:
When i have 2 users using the internet (large use) I am able to get about 42Mb bandwidth upload (DL has no issues).
However when i use one system i am only able to get about 15Mb upload(41Mb down).
It seems like the PIX it throttling the upstream bandwidth. Yes i know it doesn't do this but does anyone have a answer for why it might be doing this??

Any ideas??
Thanks for your help...