You had gains each year your basis went negative. They should have been reported as capital gains on your return for that each year your distributions reduced your basis below 0.
If you failed to report the gains you are required to file amended returns and pay the tax plus interest and penalty.
Generally, as a technical matter amounts undeclared after six years, if the amount of income undeclared is under 25% of gross income is not assessable.
As a practical matter (although not correct) is to report a negative basis on schedule D so that the gain on disposition includes the amount previously unreported. If the amounts in question are relatively small this will probably work. The cost of filing amended returns may greatly exceed any tax penalty and interest due.
