Either your hard drive is on it’s last legs, your memory has something wrong with it, or your motherboard is fixing to die.
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If the diagnostics don't indicate anything I'll clone the hdd and try it out... I don't have extra memory but I'll remove bank "1" leaving bank "0" to run on ... if it's memory it'll probably be the higher addresses as this doesn't happen until after most things load.. and then only part of the time..