The Hamilton 955 is a re-labeled 3013 Time Computer module (off the same production line, in fact).
Replacing the quartz is the first thing I'd try (preferably, with one of identical type).
Not entirely sure why you have the random missing digit...the fact that it's not always the same one, and all four digits DO light at some point, tells me it's NOT a display issue...but more likely an artifact of intermittent "drop-outs" in the oscillation of the quartz.
A fresh set of silver-oxide cells would probably be of benefit, also.