I have noticed after reading up on cpu throttling that my T21 is running in performance mode for both AC and battery. So, I recompiled my kernel with the relevant governors as modules rather than as they were - built-in - so I could see what errors were happening when modprobing. I'm trying to use 'ondemand' as the default governor. This is what I get in dmesg:

ondemand governor failed, too long transition latency of HW, fallback to performance governor

Any ideas about this? I have been over the kernel config many times looking for something that might cause this problem. I'm wondering if it's a hardware problem but I have googled for it all day and can't find anything. Thinkwiki doesn't say there is any problem with the T21s and CPU throttling so I can only assume it's kernel related.

No ideas, but some suggestion:
(1) have you tried loading different cpu frequency drivers? On my system I can use acpi-cpufreq or p4-clockmod, with slightly different behaviors.
(2) you could try using a userspace governor (http://sourceforge.net/projects/cpufreqd for instance). Many distributions should have one built-in
(3) is it the same with the "conservative" governor w.r.t. the "ondemand" one?

I found a solution - using userspace governor with powernowd does what I need.