Ah I understand
I'm afraid there's either no option to turn SpeedStep back off or it's hidden away somewhere. It looks like a custom BIOS so maybe giving Lenovo a call (or email) will help, they should at least be able to provide a workaround if not a complete solution.
Your system is quite old I believe though so Lenovo may no longer provide support for it anymore.
EDIT: One more thing, you can apparently stop SpeedStep (although it's not technically disabled) by going into the Power Options through the Control Panel and changing the Power Plan settings. Go to Processor Power Management and set the minimum and maximum to 100%. It may help, it may not, but I thought I should mention it anyway.