NobodyWiser: This first occurred when I first got Imoen after leaving Candlekeep and she becomes part of my party. After a couple seconds text indicating that I have manually cut her from my party pops up and she leaves. The same happens when I try to get Kahled and Jaheria, only they leave quicker. From what i've seen the bug can only be stopped by keeping the party members non-stop moving but this only works for a short time.

I share the game with another guy, the one who first got it and modded it. He was the first one to start a new game and his unlike my game, works fine.