Visio diagrams change when reopened.

I have a problem diagram which has been created in visio 2000 that is updated
regularly. The problem which occurs randomly is that when the Diagram is
opened the colours and shapes have changed which in turn makes some object
unreadable. The Visio file is large approx 2Gb and contains approx 20
worksheets however this is not uncommon and this problem only effects one
visio file. The file is opened from a network share however once again not
the only one. I have updated all the clients which open these files with all
the latest service packs and hotfixes but so far i have had no joy.

This is becomming a very time consuming problem I hope that some one can
help me find the answer.