SATA drive dropout

The System:
MSI KT6V-LSR motherboard
1gb Kingmax DDR 400
AMD Athlon XP 3200+
1 x 80gb Western Digital SATA
1 x 120gb Western Digital SATA
420w Thermaltake PSU
Windows XP Pro.

The Problem:
The 120gb SATA keeps dropping out. Windows explorer shows it as D and E drives but after a time it suddenly drops out. After reboot it comes back but as I and J drives.

Why is this happening and what can I do to resolve this problem?