Left side italian bb cups

Italian threaded BBs are always prone to this because the left cup unwinds in the same direction as the crank rotates so in time, they will always come loose whatever you do. This is inherent in the design. You've done all the right things, not sure what else there is to stop it recurring.

Italian left-hand (NDS), adjustable cups have a right-hand (ie conventional) thread, the same as all other bottom brackets.

The right-hand (DS), fixed cups are also right-hand threaded (as are French BB), which is where the problem lies.

Scroll down to the bottom of this page to "Bottom Bracket Threading".

Common sense would suggest that the cup is rotated in the same direction as the spindle rotates; however an effect known as precession has the opposite effect.

None of which helps the OP: presumably you'd have noticed if the bearings were shot which is the only thing I can think of.