i would be entertaining the idea that the century's engine is getting on 20 years old, running on crapper fuel here than it was ever intended to in japan, and can no longer maintain stock idle speed against the load of the highest transmission gear (reverse) against the torque converter
my bimmer was of similar vintage, and whenever i got it serviced, the mechanic would put it down to stock idle speed (750rpm or something) and it was a constantly-stalling piece of crap, even just clutching in at an intersection was enough to conk it out. i'd raise the stopper screw up till it sat on 950rpm and it never missed a beat
so perhaps just try raising the idle a couple hundred rpm?