I do not know the law that well, but from what I read I do not see any mention of having prior distributions considered....
Now, maybe you lose the lump sum definition because of that... but when did you get a distribution? Was it taxable?
I read (and of course I could be wrong) that as long as you do a lump sum of everything, including your stock, within one year you can exclude the gain....
(D) Lump-sum distribution For purposes of this paragraph— (i) In general The term “lump-sum distribution” means the distribution or payment within one taxable year of the recipient of the balance to the credit of an employee which becomes payable to the recipient— (I) on account of the employee’s death,
(II) after the employee attains age 59½,
(III) on account of the employee’s separation from service, or
(IV) after the employee has become disabled (within the meaning of section 72(m)(7))