HD Spinning down.

For a while I’ve been having problems with my HD spinning down for no reason. Right away, it spins up, makes a clunk noise, reads, and the computer locks up.
I’m using Win98se, the MB is an Asus P3B-F, BIOS rev 1006, and the HD is a Quantum LM20.6, I’ve had two different drives do this, and I’ve checked Quantum’s page. All power management is disabled, yet it still does this. I’ve tried new cables, changing the drive setup, nothing has helped.