Ladies Rolex, keeps stopping after service

[Fredrik]

A relative of mine has a nice ladies Rolex, mid 90's approximately. Historically it had no problems, but after a service last year it just stops sometimes and it is not possible to get it going again. It has gone back to the RSC a couple of times already but they never really said what the problem was. It has a two year service warranty and they fixed it within that.

Now it has stopped again, this time in the middle of the night on the night stand, so no shocks involved which the RSC sort of implied the last time.

So, does any watchmaker here have any clue of what could be acting up? The RSC implied problems with the balance spring getting tangled up, but you would need a really hard shock for that to happen? Wrong balance installed? Balance not properly, uh, balanced?

I am very thankful for any insights or suggestions, I am the one bringing it to the RSC every time and I need to have a serious talk to them this time.