i seen this happen before, the problem lied with the power saving features in the bios.

The NIC's IRQ was being monitored and whent he computer went into "low power" mode, it would shut down the NIC, but wouldnt re-enable it when the screen saver was shut off...u may want to look into updating the driver for ur network card, or go into ur bios and disable all the power saving features.

-Jay