Could it be that something is overheating? By forcing it to restart all fans out, it might keep the system cool enough not to trigger a restart. Maybe the reason why you had to replace capacitors in the first place is because of overheating.[br]
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Not a Mac expert so my questions might be stupid, but did you check the cooler / termo-compound of the CPU and/or GPU?[br]
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To me, it does sound like an overheating of the CPU. if you could monitor your temps, you might just find out whats wrong.