Overheating Toshiba A45 series

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After replacing my wife's old Toshiba A45-S120, I am now at leisure to
explore why it kept slowing down and displaying a Toshiba driver-generated
warning about cooling system failure. From Googling these words one can
guess that it's a common issue.

The popular wisdom is that the CPU's heat sink assembly is encrusted with
dust, preventing air flow. Fortunately the whole shebang is under 2 easily
removed doors on the underside. Surprisingly though I saw very little dust
in the aluminum vanes of the heat sink, but nevertheless cleaned it out
thoroughly and reseated the whole assembly on the CPU atop a brand new
coating of thermal compound. Same problem.

Tooling around the back of the laptop I find that there is a tiny second
fan near the USB ports. It turns freely if I blow air inside, and doesn't
really seem to be a dust collection point. However, with the machine
powered on, I see it stuttering and struggling to turn a few degrees at a
time. Unfortunately there is no door from which to reach this one easily
(I see it behind the rear grating) and it seems to spin on its own. This
suggests to me that it's an electrical problem with the
fan/controller/sensor which is probably not economical to even attempt on
a dinosaur of this age.

Service manuals for this series don't seem to exist; others have looked.

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