NETGEAR DG834PN Odd behaviour.

Hi all,
This is my first time posting on these boards so if this isn't the correct forum to post in, please tell me or relocate this post.

My NETGEAR DG834PN Router has recently been demonstrating some odd behaviour. As anyone who has experience with the product knows, it comes equipped with an LED dome that indicates which of the 6 antennas are active at any time. I have 3 computers on my home network, 1 hard-wired to the modem and the other two connected via WIFI. I also have WPA2-PSK encryption enabled.

What's bothering me is that I get very slow connections to most websites during the day. Ordinarily I'd blame the ISP but, even when only the hard-wired machine is on and using the internet the dome-LED setup indicates that the antennas are active and broadcasting to another machine. This is very strange because under the router-setup screen only the hardwired machine's MAC address is visible. This indicates another machine is making use of the router without broadcasting its MAC address to the machine, or is perhaps packet-sniffing for the router's password? I can always solve the LED-activity by changing the router's access password, but generally, within 12 hours or so, the problem returns. I'm next to certain this is due to a hacking and/or unauthorised access attempt from within the router's WiFi range, but short of changing the password constantly (impossible, given the other users on my network are utterly modem and PC illiterate, incapable of even altering their own password settings) there doesn't seem to be a solution.

I'm engaged in the process of resetting the router from the ground up (via hard-reset switch) and reinstalling the drivers, but I've already done this once before and it only bought me a day or two.

Any advice on what to do? Perhaps how to trace the unauthorised access, or a security measure that would be next to impossible to access without authorisation?

My thanks in advance.