If you are sure it's not the routers at fault?
Have you tried all wireless channels. Try 1 6 and 11.
Anything on 1 will overlap and interfere upto channel 5, anything on 6 upto 10 etc.
Hence manufacturers use 1, 6 and 11 as default. Give this a shot.
If not, I would install a bunch of access points around the house. Either add one or 2 more... Not the solution you are looking for but the 2.4ghz is largely unregulated and you cannot easily force someone to stop using their network because it's causing a few issues on yours.
You will also find the cause near on impossible to pin point as it could be a router, baby monitor, cordless phone, AV receiver, microwave oven, cordless 2.4ghz keyboard / mouse. The list is so long.
Edited by ukhardy07 (Sun 28-Oct-12 18:26:36)