Compact Wireless-G Adapter Connectivity Issue

Hello,

My new Linksys wireless USB adapter picks up the signal from my Main wireless G network signal, but the inernet connection is sporatic (despite there always being at least a 'good' connection). Computer than freezes and cannot get out of internet applications. I can run my laptop in same room (built-in card) and get a better reception and no connectivity problems.

Is this a router problem (main or USB adapter) or could it be computer - which is 4 years old.