Are you sure it’s not a problem before Zbrush? What app and plugin/tool are you using initially? What level of subdivision in Zbrush? How much RAM do you have in your system?
You need to provide more information for anyone to help you. It’s a consistant theme here, where everyone seems to think they have done nothing wrong and that it’s a Zbrush issue. I don’t work for Zbrush, but I do move a lot of files back and forth in various apps while working with people and haven’t seen these issues - except when someone has made a mistake. I’ve done it myself way back when. 
Are you chance following the pipeline guide for Maya or XSI that have been posted or are you doing something other? If other, then what?