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At the initial screen--in black-and-white video--choose "single-step" boot, and the system will give you painfully clear messages about every single step it is taking.
Write these things down on a piece of paper!
When it crashes, you will know EXACTLY what caused it. You may, e.g., have installed something new, and a module of its that is executed on startup is causing the problem.
It will take a LONG time to boot in this manner, but it's worth it. Instead of having to GUESS, you will KNOW with certainty.
Suggestion for future: last thing monkeyed with = best candidate for most likely thing broken.