...Then apply fresh thermal paste (AS5 = Arctic Silver 5) on top of the small square of the CPU only...
It worked! Thanks again.
By the way, here are a few details I discovered in the process that might help the next guy with this problem (on some A22p's at least). Please let me (us) know if you think I made any errors here:
1) The heavy, cast-aluminum, heat-sink/fan assembly has a stiff, but relatively thin, metal plate (chrome-plated from its look) riveted to its CPU (bottom) side. I followed your (and Arctic Silver's own) instructions in cleaning off the old, and applying the new, thermal compound to the CPU surface that contacts this plate, BUT...
2) The other (top) side of the same plate appears to be held against the heat sink itself only by the pressure of the CPU when the mounting screws are installed. There was no original thermal goo here, and thermal contact seemed problematic -- another potential cause of my overheating problems? Anyhow I smeared more Arctic Silver on these surfaces as well (prying them apart just enough with a small screw driver to insert a scalpel blade carrying the goo), in hopes of further improving conduction between CPU and heat sink.
3) On the other (top) side of the cast-aluminum part of the heat-sink/fan assembly there is a thick, snake-like, metal tube (bar?) that appears intended to help carry heat from the CPU area to the fan area of the assembly. (Just out of curiosity, does anyone know what this is made of?) There is old, white, thermal goo between this "snake" and its channel in the cast aluminum, but there appeared to be no way to replace that with Arctic Silver because the two are crimped together. Any other suggestions here?
4) Finally, after re-assembling the unit, I noticed another tubular, snake-like, tube/bar bolted to one edge of the cast aluminum heat sink and running around a corner to the main hinge on the back of the machine -- possibly another thermal conduit? If I was supposed to add Arctic Silver to that joint as well (there was no original white goo there either), I missed it. Is it worth another dis-assembly to remedy this?