inbox inaccessible when pst resides on network drive.

inbox inaccessible when pst resides on network drive.
We have many users with windows 2000 and windows xp, they
have there exchange mailbox as the primary and the
personal folder as the secondary mailbox. to avoid
getting an email from the exchange administrator they
move their information to the pst on the network drive.
when the network drives start getting upto 300mb+ the
network drive seems to disconnect from the PC or from
outlook and the mail in the pst cant be accessed. at this
point i can open the network drive just find outlook cant
see the network drive, i close outlook open outlook and
there is the mail in the pst mailbox. any ideas, any help
would be appreciated , thank you.