How would you explain, troubleshoot (and solve) the following problem?

Wifi ADSL modem router D-link 2640R installed in living room at about 1.8m height. Working fine, synchronising and getting/serving stable internet connection.

First situation:

-Laptop 01 in other end of the house, let's say in room01 southern to the living room, distant by about 15m. Getting stable signal of good to very good quality. No disconnection.

-Laptop 02 in room02 opposite to room01 (5m West) which makes it almost at the same distance and direction from the router located 15m North. Getting stable signal of good to very good quality. No disconnection.

Second situation:

-Laptop 01 moved to room03 Northern to the living room (actually just 3m behind the wall where the router lies). Getting stable signal of excellent quality. No disconnection.

-Laptop 02 still in room02 but now experiences frequent disconnections (actually almost impossible to get the Internet even though the signal level is still very good. Either no Internet with the wifi icon appearing connected to access point or no connection established at all which happens every 2 minutes and that means virtually no Internet at all as I can just get a timeframe of 1 minute or so to load any website or even get to the router's web based control panel. If Laptop 01 is completely shut down or its wifi adapters shut down or even still working but its wifi MAC address forbidden, then Laptop 02 has no problem at all. If Laptop 02 is moved to a nearer location to the router, in the living room for instance, then no connection problem occurs even if Laptop 01 is also connected. And also if we move back Laptop 01 to its original location (room 01), then no problem as well.

I'm completely lost and don't know how to address this issue. I tried to change the Wifi channel and even tried the auto channel scan but that didn't solve it. I know that the problem is probably coming from Laptop 01 being in its new location or some sort of interference as the problem occurs only under the described condition but I have no idea how to solve it!

I also scanned the neighborhood for wifi jam using InSSIDer, there are few other access points but they don't seem to affect the situation.

Any ideas about the steps to follow or tools to use ?