Is this an Italian bike? Italian threaded bottom brackets have right hand threads on the right hand (fixed cup) side, which, because of the direction of rotation, can loosen themselves. If so, be sure your mechanic uses Loctite on the right side threads. I'm still confused if you have a sealed cartridge bottom bracket or an adjustable cup-and-cone bottom bracket with loose bearings. When you took it apart, did the bottom bracket come out as one sealed unit (spindle inside a cartridge) or as a separate spindle, cups, and ball bearings? It's not normal for a bottom bracket to need repeated tightenings, unless it is Italian.