Let's get this straight,those 15 metres the signals need to cover are being disrupted by various other disturbances ....simple logic.
1.Ok,first move off any cordless phones,wireless radios out of the room as they might interfere with the signal ,or just change the channel in which the wifi operates.
2.Make the position of the access point strategic,that definitely does not mean the small empty space in the corner of your house.It should be a central point,easily accessible.
3.Upgrade your router's firmware if its an old one,It would definitely improve reliability.
4.Install a high gain antenna onto your router(though some routers do not support this).Though some people say its illegal,but i think its the power output is the matter of 250mW,the linksys gets nowhere near it so i dont think that actually illegal.
Thanks roopak for your comments.
Now watch your signal strength soar....