The usual - slow wireless connection

I know this topic has been discussed, but I can't find any help anywhere!

PLEASE HELP!
Here's my story:
I bought a Buffalo wireless router, along with a USB adapter thinking they'd both work fine together.
Everything seemed to be fine (top speeds), but now all of a sudden my wireless connection seems to be slow. The speed can vary from anything up to 150kb/s to 10kb/s, and my actual speed in 512kb/s (using a wired connection)
My upload speed never changes - it's always around 20kb/s which is the wierd thing.
I've tried removing / re-installing the driver and 2 different wireless clients, re-initialising my router, flushing dns - ipconfig...

does anybody have any ideas?!

Thanks

I was wondering.... I have a Buffalo WHR-HP-G54 and was just wanting to check that it needs to be connected to a router. I've read in some places, that some wireless routers do not function correctly if it's connected to another router.

Any ideas?