Like many here, I was experiencing random hard lockups in FS9. The lockups that result in frozen display, stuttering sound, and no way out except hitting the reset or power button. It got much worse when I recently installed a new AMD Barton processor and upped the FSB from 133 to 166 MHz. I couldn't complete a single flight without a lockup. Sometimes it happened before I left the runway.I tried all the fixes that have been mentioned here and nothing worked. Finally last night I noticed that the DDR voltage in my BIOS setup was 2.5v, the default on my Soyo MB. In poking around the Corsair memory support forum, I noticed that they recommend 2.7v, and will still honor the warranty up to 2.8v. I changed the DDR voltage to 2.7v in my BIOS, and bingo! I was able to complete two flights without a lockup. That's no guarantee, but before upping the DDR voltage I couldn't complete a single flight without a lockup. So far it looks promising.

Day 2, several more flights and still no lockups. I might be onto something here.

Thats great news. I hope that people who have been experiencing these lockups see this post, because this is a very hard to figure out solution :)