A global tax will do nothing but hurt the world as a whole, not solve any currency instability internationally. Countries who are doing good should not have to help countries that are doing bad. Countries that are doing bad do not deserve to be help out by the globe, they need to solve their issues on their own.
No, a global tax would not be an effective way to help solve the currency instability internationally, because it would only lead to redistribution of wealth. There is no economic evidence that a tax can solve currency instability. In fact, a tax would be more interference with market forces, and would likely only cause even more instability.