well, I guess the question is "what part is coming loose?".
The candidates are:
1. adjustable cup
2. fixed cup
3. crank arm is coming loose on the axle, which gives the impression that the bearings aren't adjusted properly.
I'm guessing that you have to adjust the left cup to tighten things up, which wouldn't point to the crank arm coming loose.
Is the lock ring loose, or do you have to loosen it first before adjusting the left cup? If it is still tight, that suggests that the right cup might be backing out.
Photos would help, as always.
A video of you checking on the looseness and tightening things up might not hurt either.
Steve in Peoria
(I'm betting on the right cup coming loose, just in case it wasn't obvious
)