Can you temporarily hard-wire your computer to the router/internet long enough to see if anything changes in your web-surfing?
If that works well, do you have any other computers (laptop etc) that link to the router wirelessly?
If another computer has signal issues too, you may have a bad router.
And if another computer has no issues when it's connected from the same general area your suspect rig is, then it's the adapter, USB port, or maybe even a software issue. (new driver etc)
The adapter is at the top of the suspect list, but if you can test a couple things, you can zero in on the bad part.