I haven't tried safe mode as there hasn't been much commonality with the time. One time it's been on for 2 hours and crashed while I'm in Open Office, but there are days when I run my games (Tera and SWTOR) for 10+ hours and nothing has happened at all. I've had 3-4 days between a crash - then days like the day before I first posted this it was 2 times in one day
There is no loud sounds, changes in fan speed, funny smells, etc. The monitor turns either solid yellow or it has blue and gray vertical stripes. The computer does not shutdown, but it will not respond (that I can see) to any input. I tried to shut it down by pressing ctrl-alt-del twice as a test to whether it's still working behind the screen, but there was nothing. Not sure if MS took that out between XP and 7 though. To recover I have to restart the computer from the tower otherwise it will just sit there.
It's crashed about 8 times now. I have changed nothing inside the tower since it was put together in August. Nothing around it has changed, it's never moved from where it sits on my desk and I blow the dust out of it every couple weeks.