Mac Pro :: Seemingly Random Freezes In Leopard 10.5.4

In the last week or so i have been experiencing lock ups every 15-20 minutes, which last for around a minute. The problem has only presented itself after I moved to a dual monitor configuration, although this may be coincidental, given the nature of the problem. The problem only seems to occur when the system is reading from from the main hard disk however testing the disk (and performing a directory/file test) says that the disk (and all others in the system) are functioning properly.

Curiously, any audio playing at the time, regardless of the application it originates from, continues to play while the rest of the system is unresponsive. It also appears as if the keyboard and mouse are disconnected during this time, and the power lights on each begin to flash, and the Caps lock key no longer lights up. I would like to avoid replacing the system disk if at all possible, as i have already had to do this three times since last December, when i purchased the system, due to drives developing faults.

2x Dual-Core Xeon 2GHz
1GB RAM
NVidia GeForce 8800 GT