the way I read it, his "spirited driving" holds the temp fine, it's just when he slows down to a crawl after such speeds that the temp climbs!
that sounds like a fan clutch to me. the original is VERY PRICEY, but the updated one from the 606 is pretty affordable.
to test the fan, go "spirited driving" and then park and turn OFF THE CAR!!! immediately pop the hood and see if you can freely spin the fan, or if it is very firm.
it should be firm and not able to spin by hand more than 1/2 turn or so. if it freewheels, you have a problem.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!