If a mechanical watch is running without stopping, this still doesn`t mean that this watch is OK. Not even timekeeping shows the health of a watch…
There is a parameter - Amplitude of the balance oscillations, that actually shows the condition of the watch. When the watch is OK, then amplitude should be 250-330 degrees, equal in both horizontal positions, and a little less, but again equal in all vertical positions.
So, if somebody new to the world of watches, asks "Why my watch is stopping sometimes?" , the first thing that a watchmaker will ask, before give any answer, is " And what is the amplitude???"
If the amplitude id good, probably the watch will run with stability and no stops because of not enough energy coming to the balance will happen.
If the amplitude is weak, then the watch may stop easily in many situations, and good timekeeping will be impossible to achieve…
Weak amplitude means, that not enough energy comes to the balance from the mainspring thru transmition and escapement, or there are losses of energy in the balance itself…
Not enough energy can be because of bad or dirty bearrings, worn pinions, weak mainspring, worn or badly adjusted escapement parts…
Losses in the balance can be because of bed or dirty bearrings again, also bent hairspring, and improper collaboration with escapement…
If and old watch, that have not been running from many years, will have to be brought to life again, the first thing to do is to disassemble it, clean all the parts from old hardened oil, and inspect all parts for signs of ware. If needed, some parts must be repaired, replaced, or made by the watchmaker if no replacement is possible… Then all must be assembled, oiled, regulated…