Had the same problems on my wifes.
It was hard to steer right and sometimes left.........before I got to fix it she broke her steering rack <img src="/forums/images/graemlins/zombie.gif" alt="" />
After we replaced the rack (which was leaking fluid pressure past the seals)it steers fine now.
So in our case we eliminated the pump,lines and level as the cause when we replaced the rack.
As for the fluid,what little bit leaked out during the install was minimal and it wasn't burnt etc. it took less than two quarts to fill it up.

I'm not sure if it's that your slowly loosing pressure(past the seals)or maybe your fluid has broken down.

I would change the fluid a couple times(to get rid of residual fluid in rack) and try it it isn't hard to do,just a little messy.
Good luck

Bansil