Its pretty simple, your router is probably a old B/G standard non MIMO wireless router. Your floors maybe concrete between them or one of the million possiblitys of EMI.

Options

A. buy a wireless extender stick it downstairs.

B. buy a MIMO wireless router, reccomendation Netgear DG834PN (solution may not solve the problem as the EMI maybe too strong but this exact router covers from the 3rd story, all the way round the house, on the bottom floors through think walls and floors. It also reaches to the bottom of my garden, which is another couple hundred meters and outside the front of the house.)

C. connect via a wire.

D. try and find any strong EMI points, aka hoover, halogen light, radio etc...

E. Move the router downstairs.

F. shout this is sparta.