Getting this thread back alive because i have the same problem! Running windows 7 64bit with a linksys WMP600N card and installed latetest Ralink drivers. Signal is most of the time 70% but surfing speeds spikes all the time, one sec good speed 2 secs nothing etc ... just about the same...
Seems like windows 7 isn't liking your usb wireless adapter, would be better maybe to change it to a well known brand to keep you spared from future networking issues ... To be honest, i never heard of that brand of your wirless usb adapter you where talking about, but then again i'm from Europe.