There are as many potential solutions for interminable PC shutdowns as there are potential causes, so it's best to start simple before resorting to Registry tweaks and complex diagnostics.
Last year's police shutdowns of cell phone service in San Francisco subways was prompted by protests against police shootings. The FCC wants public input on the issues around shutdowns.
BART can shut down cell service only in the face of unlawful activity that threatens safety of people, property damage or subway service disruption.
Faulty wiring or power supplies could be to blame.
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Apple service document discusses MacBook sudden shutdowns