Toyota Repair: Overheating Previa, radiator fluid, radiator shop

Question
The Previa has been running a little rough at idle for 2 days, and today a huge amount of white smoke poured out of the exhaust pipe and then the engine stalled.  I coasted to a stop and have not restarted it.
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Followup To
Question -
Our 1995 Previa LE/SC recently has begun to overheat only when it is under load (going up hills and it really overheats up hills when the outside air is hot).  All other times the temperature is rock solid in the safe zone and doesn't budge.  A dealer has checked the head gasket and said there are no problem.  The radiator fluid and engine oil are at the right levels.  Any ideas what the problem is?
Answer -
I recommend that you remove the radiator and take it to a radiator shop to have it checked, it may be partially clogged and either needs to be cleaned out or replaced.

Answer
A huge amount of white smoke would indicate some sort of an internal combustion leak, possibly a head gasket or even a cracked head, if the engine still runs the repair shop should be able to do a block test to determine if there is an internal leak.