overheating (again)

[sd88supra]

after replacing my water pump, installing a 2200 cfm shrouded electric fan, changing (and topping off) my coolant, getting a block test (no blown or leaking hg) and tightening my serpentine belt, my car still overheats after VERY LONG idling. It does run cool under 'normal' operation. Even during heavy traffic it stays cool, but when crossing the U.S./Mexico border and waiting up to 2 hours to get through Customs, my temp gauge gradually starts to go up. Never quite reaches the red zone but does get close. Turning my heater on helps but it's San Diego! There is no such thing as winter here!

Once I get passed Customs and hit the freeway my temp gauge drops quickly. Makes me think its my water pump (or the pulley), but then again when I did have a bad water pump my car got hot quickly right after stopping. This is a gradual thing. Any ideas??? I was thinking this is normal (after all, cars aren't designed to idle forever). And its like a gamble, cuz it doesn't always do that. But it has more of a tendency to do that after driving for a while (and pushing it hard). It's an 87 N/A.

I know, I shouldn't be driving my baby in Mexico (or should I? ). I'll stop once I get it painted hehe...