Atmospheric corrosion of the older (Macintosh to Plus) logic boards is a real possibility, as also are cold solder joints. Try an inspection with a good magnifier, looking especially for unlacquered traces with orange or green discolouration, and dull or cracked solder joints. Begin at the system-identified RAM chip, but include the whole board. The board can be either spot-repaired, washed and re-lacquered, or given some combination of these, especially if it is a primal 128K that you have and wish to retain in (relatively) pristine state.

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