Inter-adapter interference?

OK, am struggling with a domestic spread-out network from a Belkin pre-n router, with three wireless PCs and one wireless laptop, all using 'g' adapters of assorted makes, plus one PC wired to the router. All working well except the one furthest away (forty feet, two rooms = three walls and the wireless laptop intermediate between them). This rogue PC gets a good signal most of the time, but drops connection rhythmically, timing out and often breaking secure internet connections. I have changed channels, and done all the obvious things without anything making much difference. Tonight I was playing around with the rogue PC and the laptop, and noticed that, running a ping test continuously on both, whichever one is positioned nearer the router gets consistent fast response times and 0% packet loss, but the one further away drops packets every 15-20 seconds (5-8% loss). It seems that the nearer adapter interferes with the further away. Is this possible? They are almost line of sight. Any solution other than moving one of them (not easy, for practical reasons)? TIA for any advice