I had a similar problem at home, I have a Belkin wireless router and linksys cards in my laptops and they had a signal of 70& or over but the kept losing the signal at irregular intervals

I had the router wireless mode on Long Range Mixed 11b + 11G, I tried changing the mode to just Mixed 11b+11g and the problem stopped.

Im not quite sure what the difference is between the modes (other than the obvious) but I think the long range mode made the signal susceptible to outside interfearence??

I didnt look to deeply into the difference, I was happy as long as it worked!