Linksys Wireless card issues

Hey, i just recently built a new computer and have everything running smooth!!
The only problem is my wireless card, i have a linksys wireless router hooked up to my comcast cable modem. I have three wireless computer (including this one) and one wired, which the router is connected to.

So my problem is i can connect to the router with my wireless linksys card just fine, i have an excellent signal strength 24/7 as well...but whenever i use anything internet based (downloading programs, using firefox, etc...) the speed jumps from 54.0mps down to 12mps and sometimes lower. I have another computer in the same room that has excellent signal strength and is always 54.0mps, no matter what application is running.

So what can i do to fix my problem...

Thanks!