Gas and Oil Home Heating Furnaces - Condensation on Walls

Help... We've recently noticed that the walls of the hallway (in the center of our house) are dripping with condensation. This could be due to two things, I think. First, that our A/C doesn't cool the upstairs properly, and the temperature differential is causing condensation. Or else the exhaust fan in our bathroom is sending damp air into the wall. Two questions: how might we circulate air upstairs to prevent condensation (we can't affort a second unit and the upstairs is really a converted attic, and is one room only) and if the bathrm. exhaust fan isn't working, how can we tell that and then fix it?

I've really been scratching my head over this one. Condensation is caused by warm moist air hitting a cool surface and then condensing. (I know, we knew that -- I'm just thinking this through).
How old is your house?
Does it have hollow, open walls to underneath the house (popular design in the 1800's)
Is the condensation forming at the top of the wall or all of the way up and down the wall?
I'm thinking if I knew a little more I could help you figure this out.

Thanks for your reply. The condensation is coming from the top of the walls. Our house is circa 1920 (plaster walls). Our current theory is that the exhaust fan from the bathroom (a new room which was added a few years back) isn't routed to a vent on the roof or an exterior wall. It's ducted into the attic... So warm, moist air's been blowing into the attic. What we've been told so far is that we need to re-route the fan (how difficult is this?), wash the condensation down with bleach, and get an ozone generator to blow into the attic for several days to kill whatever mold might have formed in there. Does this seem like a reasonable solution?