Dropping CIFS shares in sleep mode

I have a problem on my Macbook Pro running 10.5.7 (but this was also happening on 10.5.6). I attach to an OES2 Linux server via CIFS. This has worked well for months (I've had the Macbook for 20 months now), but in the past 3 months or so something odd is happening. Frequently (but of course not always) when I close the lid and put the Macbook to sleep, when he Macbook wakes up it will have dropped the CIFS shares (there are two of them). Sometimes this is evident by the volumes no longer showing mounted, and other times the volumes WILL show mounted, but will not show any files when selected.

Nothing I can do at this point will reconnect the volumes (short of restarting, which always fixes it). I can try to mount them through Finder. I can try to mount them manually from the terminal.

At the same time, our iMac is connected to the same volumes and working properly.

Restarting the Macbook always fixes the issue, but that's annoying of course. Does anyone have any bright ideas on how to best troubleshoot this, or what might be occuring?

Thanks.

Danita