Precision T7400 will not boot with SpeedStep

If I reset the CMOS using the manual procedure of applying the password jumper, and then enable SpeedStep in the BIOS, the computer will appear to POST successfully and initialize the LSI storage controller successfully but will then hang (no keyboard numlock response, nothing) before attempting to boot any volume. In this state, about 80% of the time it will be impossible to enter setup via F2. Resetting the CMOS again solves the problem, as SpeedStep is disabled by default.

To attempt to narrow the problem I ran a full memory test from a Linux stick and there were no errors; I disconnected all but the most critical peripherals. This hang still occurs with no connected volumes, and nothing but the USB keyboard and the video card.

I'm somewhat at a loss. With SpeedStep disabled, there's a noticeable drop in performance, typically for inefficient web apps.