First of all I am not dead, just don't have much time these days

Occasionally when we import a PDF into Indesign and put Smartstream Text Box Channels over it they look fine on screen. But when they print they are a 1mm or so too high.

This doesn't always happen, deleting the text box and redoing the Channel normally works. Is this just random or is there a workaround for this?

5000 Press, Smartstream CS4