That's a tough one without seeing the file, but one thing that may be worth a look comes to mind; since the problem seems to happen when gravity kicks in, you may want to check the Solver Scale. When solving for gravity, Maya uses a scale based on real world physics so if the character is not built to scale, use the Solver Scale setting in the attributes of the solver node to compensate for the difference (so if a character was built with the default grid in centimeters and is around 20 cm tall, you'd have to enter a value around 10 in the Solver Scale to get gravity to calculate right). just a thought, sorry if it's not new information or you've already checked. (If the garment is just really high in resolution, you could also try using multiple solvers)