Does the router have anything near it that can interfere and/or diminish the wireless signal coming from it?
Things such as mirrors, cell phones, cordless phones, UPS, Ham Radio and neighboring wireless signals can interfere with the router
Also as a last resort, you may want to try this router in a different environment (i.e. a different home) if at all possible and see if the same problem exists there. If it does, most likely the router needs to be replaced