I recommend trying Single mode G or N or mixed G and N to test. A and B are not need much. However you could try those modes as well and see. I am using dual bands, the 2.4 GHz is G only and I have tried it at channel 6 and 11 so far with problems on both channels. The 5 Ghz is N only and using channel 40. I haven't noticed the problem with the N. I had my laptop downstairs for 3-4 weeks without issue. Now I have it upstairs and the issue with N is the signal cannot be maintained. This seems like this problem might be related to placement of the router. How far away is the router in relation to upstairs or where ever you have the router placed. By your description here, it seems to be that this is a WiFi issue and placement. FYI, 5Ghz works best in line of site and or maybe one wall away say in the next room. Walls, floors and obstructions can effect how the 5Ghz band works since it's such a high frequency, it's susceptible to those items. It's the nature of the beast when it comes to 5Ghz, all routers suffer this. I'm also wondering about the building materials in the house that might effect radio signals as well. I would test the following, Single mode G at 2.4Ghz only for all devices and see how it goes. Turn off 5Ghz in mean time. After that I would place the router upstairs if it's possible or in a more centrally located place and do some testing there as well. Again try Single mode G, then switch to N at 2.4Ghz and test. If your able to move the router, test the 5Ghz after the move.
As your testing the Wireless, see if you can connect your LT or your girlfriends wired to the router so you can check the wired connection. I presume the wired will be stable here.
Keep us posted. Hope we can help figure it out.