I was sometimes getting the Windows pop up telling me FSX had stopped responding and task manager would then close it without giving me the option to keep waiting.
I may have found and fixed the issue on my system. At least it seems so this far.
I had a lot of planes I had picked up over the years. Besides cluttering up the airplane menu and making that slow to open, I think it was part of the problem with the FSX startup crashes.
I did an inventory on my airplanes and compared the planes in the SimObjects directory with the planes that showed up in the airplane selector menu and found a few that weren't showing up. Turns out they were planes I had copied from P3D that just don't work in FSX. I deleted those outright and moved a bunch of others into a holding directory and now FSX loads quicker, the airplane selector menu doesn't take as long to come up, and I haven't had task manager crash it out since.
May or may not be a root cause but so far it may be. I think the P3D planes just slowed things down but it wasn't enough to cause the crashes until I got enough more airplanes to hit some threshold.