I would also suggest changing the battery on the motherboard. If your present battery is starting to fail, the system may not be retaining all the BIOS settings at times. This may account for the system booting Ok occasionally. For a $3 investment, it is a cheap item to rule out as the cause. A new 3-Volt, CR2032, coin cell battery is available at any Radio Shack, computer stores and most discount stores like Wal-Mart. You can click here <ADMIN NOTE: Broken link has been removed from this post by Dell> for additional instructions on how to change the battery from your model's on-line Service Manual.
I would suggest posting your question in the Dimension - Upgrade Hardware forum. That would be a better venue in which to connect with those who are familiar with your system and how easily parts can be swapped out for off the shelf products or parts from another Dell model.