A depressing update - problem still with me after being 'on-line' over a day so it could be a heat problem? I thought I was 'there' today when I cleared 2 kg of dog hair out of my bios chip fan duct but on return from my hospital visit of 'dad' I found the disconnect still there.
I have noted mav64's post, and indeed I have one of those 3Com routers but I'm pretty sure the link is good on the laptop when this one goes flakey. Still looking for that 'common point' between USB-RJ45 link and wifi link which would cause both to 'wobble'.