This is the first year i'm growing Fish Pepper so can't say much about stableness or so, but my plant only started expressing the variegation with the sixth or seventh pair of leaves or so, everything before that was pure green. At first, the variegation was very subtle lighter green spots on the leaves, and now that the plant has gotten bigger the variegation has gotten more evident:
(Got the seeds from a fellow finn, no idea about their original source.)