Volvo S60 Gearbox & Transmission Problems
Problem with Volvo automatic transmission
The problem you describe is a relatively common one with the Aisan auto Volvo uses. Volvo's fix is to reflash the TCM to alter the shift points, which seems to be what your dealer has done. Hopefully you won't have any more problems with it, but keep an eye on it.
2001 Volvo S60 2.4T auto transmission
Servicing the auto transmission is a good start and I would support doing that, but it could be something deeper that is causing your problem. It could be that it needs a rebuild, but you won't know until the mechanic checks it. Whatever it is, it needs addressing.