wireless adapter

I bought D-Link wireless adapter for my PC. For the first several days, it worked fine, at 11mbps. But some day it suddenly ran at 1 mbps, and usually dropped connection. I uninstalled and reinstalled it several times, it is the same. Appreciate your help. (I am using comcast broadband, Motorola modem, and Belkin wireless router. I have a laptop using Belkin wireless adapter, which works great.)