You mention you have an on-board video card.
Your computer might now be running Dual monitor mode, meaning although your monitor is hooked up to the add-on Nvidia Geforce 9400GT video card, your on-board Intel video processor may still be processing video. Since it has less memory and slower video frame rates, it can cause bottle-necking and system crashes to occur.
Check your BIOS setup utility to see if there is an option to disable the on-board video processor or check your motherboard manual for more details.
Edited by MrBruce1959, 13 February 2010 - 03:19 AM.