This doesn't sound like a saving problem, but more likely a corruption or end of file problem. I would take another look at the cell library that you ported over from V7. It may be that somewhere there are elements that V8 doesn't like or somehow the cells are being placed beyond the end of file indicator (does V8 work that way?)
Step one - place some cells and redraw the screen. If the cells are still there, proceed to step 2.
Step two - compress the drawing (does V8 have that?) and redraw the screen. If the cells are still there, you're beyond any help from me and I suggest contacting Bentley.
If the cells disappear after compressing the file, then they are somehow ending up byond the EOF indicator. The simplest fix is to place all the cells in the cell library into an empty drawing - drop complex on all of them, and recreate the cells into a new cell library directly in V8. If anyone has an easier and less time-consuming fix, please speak up.
Rob