How many chars are you trying to run?
Do you have a single/dual or quad core cpu?
What are the settings on each instance?
Does this happen on a random window, or always the same window? And is it or is it not the last opened window?

At first glance, it seems like a core problem and/or cpu failing to run all instances proper.

If so, asigning cpu cores to each window might improve. Setting the settings lower might help as well.

But that's all guesses, need to know your hardware really.