|It is true that the Wireless Access Point (WAP) built into combo routers will very in signal strength depending on the manufacture. The other problem with multi-floor houses is that metal will block some if not all signal and the duct work between the floors is usually made of metal. There is also distance factors which will weaken the signal and cross talk from other devices on the same signal. Cordless phones also use 2.4 GHz which will cause problems. Try getting 5 GHz cordless phones if this is the case.|
Possible ways to solve this problem is to see if you can not relocate the WAP closer to the computer that is upstairs. If you can not then you may want to look into getting a directional antenna. Standard antennas transmit the signal out all sides and you lose half of your signal strength. A directional antenna will send all of the signal one way doubling your strength. You may also look at getting a router that does not have the WAP built in and get a separate Wireless Access Point. They are usually strong than the combo devices and with a long enough Cat5 wire you can relocate the WAP closer to the computers that need it.
Hope this helps.
If your neighbors are also using wireless then you may be picking up some cross talk from them. If you have a laptop, there are some software out there that will show you the signals around your and you can use it to see where the signal is weakest in your house.