Well, I took it in today to the Genius Bar, and luckily, it acted up immediately. I explained that I had updated through 10.5.6, 10.5.8 and finally 10.6, so the drivers should be updated. It seems to be a hardware issue (to me). Although, I did read an article at
http://www.pbcentral.com/news/viewnews.cgi?id=EkklupyyZARGdcTbdY today that says the drivers could still be the cause (due to trying to handle the dual GPU setup).
The Apple rep talked to his manager, and they are ordering me a replacement MacBook Pro. Apparently, you cannot order parts for a refurbished computer, to repair (company policy?). Anyways, I should have a replacement in 5-10 business days, hopefully. Glad I brought it in, and here's hoping for no problems with the replacement machine.