I'd try other USB WiFi sticks.
I recently got a new PC and purchased a USB wireless adapter to go with it. I plugged it in and the internet works fine HOWEVER, on testing I discovered that I was getting very slow speeds to what I'm used to and the internet drops out quite a bit even though I have 5 bars and low ping (20 ms).
I then went back to my laptop (which has a built in wireless adapter) and confirmed that my laptop was getting 3X the speed and never dropping out - in the same room as the PC, again with 5 bars.
I have tried everything I can think of from flushing the DNS cache; flushing the IP; resetting the adapter; reinstalling the drivers; repositioning the wireless router (even with 5 bars anyway) etc.
Any ideas and help would be MUCH appreciated as this is very annoying and weird!