You said it became loose after you had just repacked the hub.
Possible that during reassembly you didn't tighten the locknuts against the cones well enough; thus loosening up?

sanity check to be safe - you did do this using propor Cone Wrenches, right? (hey some people don't know this)

k then,
dissassemble
check axle, cones, and locknuts for damaged threading that may be causing loosening

check cones and hub races for pitting