Why would this plane be flying around so long with this known fault?
because the exakt fault was
not known. Spurious computer errors do happen frequently. If resetting solves the issue, no further action is taken. If the fault can not be reproduced on ground (e.g. due to a temperature related problem, like thermal stress on a cracked solder joint) the units are declared "serviceable" and end in an aircraft again. Just when the frequency of one specific fault indicates a real issue which can be pinpointed to one specific system, more action is taken. This would have probably been the case after this flight, on whicht the unit would have failed 6-7 times or more...