I found that I could boot the computer reliably if I turned off ACPI (acpi=off) in the boot string, unreliably if I set acpi=noirq. I now can boot the computer (apparently) reliably with virtualization disabled in the BIOS.
Thinkpad T520
Linux lambert 3.5.0-22-generic #34-Ubuntu SMP Tue Jan 8 21:47:00 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
I found that I could boot the computer reliably if I turned off ACPI (acpi=off) in the boot string, unreliably if I set acpi=noirq. I now can boot the computer (apparently) reliably with virtualization disabled in the BIOS.
Thinkpad T520
Linux lambert 3.5.0-22-generic #34-Ubuntu SMP Tue Jan 8 21:47:00 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux