Many systems implement throttling based upon power draw and/or thermal conditions. You don't have to be exceeding Tjmax for this to occur. It's hard to say; we are talking about a processor that is 11+ years old. All sorts of issues, especially having to do with power and thermals, could be affecting it. Have you replaced the TIM (paste) between the processor and heatsink lately? This stuff dries out after a few years and needs to be replaced (your system is old enough that I could describe this a regularly-required task at this point).
...S
