I never REALLY tighten the screws. I probably do the same thing you did, make sure the screws are tight, normally. Now I guess you know the clamps that are flat that attach in the middle of the motherboard are what attaches the fan to the motherboard. Double check those and make sure they are tight. Double check the screws on the fan itself. Could you have loosened a screw or two on the fan housing?
As far as the heatsync, I think you would have noticed if it wasn't attached correctly.
And I'm guessing you have cleaned out all the dust, so the fan is not working harder to circulate the air....
As far as a diagnostic test, I don't know of one.
If I were you, I would open it up again, disassemble it down to the motherboard. I wouldn't lift the motherboard off the processors, that may cause it to YLOD again. Reassemble it paying close attention to the fan.
Let me know how it turns out and what you did to fix it.
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