Sounds pretty odd indeed. I know my wife's car will steer straight down the road some times and then at other times it will pull like a bitch. Typically this kind of stuff is attributed to crappy or worn out tires. Even a good set of Michellins at the end of their run will cause all kinds of funky steering and handling issues. Get your front end checked out and rule out anything there and if the tires look half way decent then get them rotated and balanced and see what that does for your situation.

Mark