Attach save from shortly before freeze occurs and the matter will be investigated.
http://www.mediafire.com/?mduydyz2b4m
This is the save. Sim who lives alone is sleeping through the night. If I hop him back into bed after he is awakened by the message that the carpool will arrive in an hour, I can sometimes replicate the freeze. It seems to always happen at 7:26 am, whether he's still sleeping or running out of the house while the carpool honks in front of the house.
You mentioned a memory issue. I have only 2 GB of memory installed on this XP 32-bit system, if that could be an issue.