It's possible that there's radial play in the threads where the cup meets the frame. This has become more of an issue since cartridge BB's replaced traditional cup/spindle BBs. If you installed it yourself you might remember if the thread was very tight (correct) or loose allowing the cups to thread in very easily. If loose, the entire BB might be rocking vertically within the BB shell.
The fix depends on how much play you need to take up. For not too much paint the cups' threads with latex paint (any color will do) letting it fill the roots slightly, and let cure for a day or two. (Do not use an enamel paint). Install the cups which now should meet some, but not too much, resistance. Excess paint will shave off, leaving you with a tight installation.
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