i have my rubber trees in those self-watering "plant spas" that you can get at walmart. it really sounds root bound. and like it needs a little more light, and some fresh soil.
i'd get a pot, just 2 inches larger (it is probably in the 6 or 8 inch pot from the florist, measure before you shop) and take it outside to repot. turn it on it's side, gently tap it out of it's pot, and look at the roots. i bet they are circling it.
if you can, pick at them gently just to free them up some, cut off the spiky ones with a clean sharp knife or shears.
then, put an inch or two of fresh soil in the bottom of the pot, put the plant in the center, add soil around the edges, you will want to "water the soil in" so that you get it "full". fill it to a couple of inches of the top. then water when it starts to dry.
sometimes, they need more water than others.
and, if you can move it to a north east or north west (or east) window so it gets just a bit more light, it should be very happy.
i've had them in west windows and they do fine. i've had them in full sun and they did fine. they just don't like to get frozen!
(i've done that to them too!)