If you shoot in M or Tv mode, you always can manually set shutter speed to 125. In Av and P mode, the camera will set the speed between 1/60 and 1/125, depending on the chosen aperture and the ambiance light. So you will only get 1/125 in Av or P mode if you are shooting at bright daylight, when the camera decides to do fill-flash. As in P mode the aperture is chosen automatically by the camera - depending on what kind of ISO film you are using - and this usually won't be the one with the largest opening, you will rarely see 1/125 flash-sync in P mode. Using Av mode at least allows you to manually set the largest aperture and thus forcing the camera's automatics to choose 1/125 sync-speed as soon as it's programming allows it.
Remember, all these routines only work with a dedicatet TTL-Flash (I don't know if the Sunpak belongs to that category). If the flash is not TTL, you would have to dial up 1/125 in M mode, choose an aperture and set up your flash accordingly.
Hope this helps, as the manual is quite unclear about it.