yep, my network's password protection, and encryption is on - but i can't decrease my range, because my partner works down the other end of the house, which is actually further away than my nearest neighbours.
i don't think it's the cable speed fluctuating - it's actually like there's heavy traffic, but there's seriously nothing else happening from my computers - no mail, or downloading or anything like that.
it's happened a couple of times before, and resetting the base station has helped (also, I've just uploaded the new basestation software, so maybe that will make a difference.)
thanks for your suggestions, and i'll post again if anything else occurs to me.