Buddha's past lives in the Pali Suttas outside of Jataka According to this answer for the question "Does Theravada Buddhism accept Jataka Stories?":


  The Theravada accepts the Jataka stories as commentarial literature;
  they are not canonical, but are considered a reliable account by a
  learned Buddhist scholar. What are canonical are the verses that
  accompany the stories ...
  
  It's not uncommon for modern Theravada Buddhists to question the
  authenticity of the stories, especially given their often fantastical
  content.


Based on the above, it looks like the stories of the Buddha's past lives from Jataka are merely commentarial, and not canonical.

However, I found one possible story of the Buddha's past life as told by him in MN 83, which is part of the Sutta Pitaka, but outside Jataka. Being part of the Majjhima Nikaya, I take it that this is canonical.

It's a story about the just and principled King Makhadeva who started the practice of retiring from the throne at an advanced age to become an ascetic. He practiced the Brahmaviharas, and was reborn after the break-up of the body, after death, in the Brahma realm. He established and passed on this good practice to his descendents who carried on doing it, till King Nimi's son Kaḷārajanaka who stopped this practice.

The plot twist is when the Buddha tells Ananda:


  And having developed the four Brahmā meditations, when his body broke
  up, after death, King Nimi was reborn in a good place, a Brahmā realm.
  But King Nimi had a son named Kaḷārajanaka. He didn’t go forth from
  the lay life to homelessness. He broke that good practice. He was
  their final man.
  
  Ānanda, you might think: ‘Surely King Makhādeva, by whom that good
  practice was founded, must have been someone else at that time?’ But
  you should not see it like this. I myself was King Makhādeva at that
  time. I was the one who founded that good practice, which was kept up
  by those who came after. But that good practice doesn’t lead to
  disillusionment, fading away, cessation, peace, insight, awakening,
  and extinguishment. It only leads as far as rebirth in the Brahmā
  realm. But now I have founded a good practice that does lead to
  disillusionment, fading away, cessation, peace, insight, awakening,
  and extinguishment. And what is that good practice? It is simply this
  noble eightfold path, that is: right view, right thought, right
  speech, right action, right livelihood, right effort, right
  mindfulness, and right immersion. This is the good practice I have now
  founded that leads to disillusionment, fading away, cessation, peace,
  insight, awakening, and extinguishment. Ānanda, I say to you: ‘You all
  should keep up this good practice that I have founded. Do not be my
  final men.’ Whatever generation is current when such good practice is
  broken, he is their final man. Ānanda, I say to you: ‘You all should
  keep up this good practice that I have founded. Do not be my final
  men.’”


The Pali version side-by-side with English:


  I myself was King Makhādeva at that time.
  Ahaṃ tena samayena rājā maghadevo ahosiṃ.
  
  I was the one who founded that good practice,
  Ahaṃ taṃ kalyāṇaṃ vattaṃ nihiniṃ, 
  mayā taṃ kalyāṇa vattaṃ nihitaṃ;


Questions:


Is my interpretation correct that the Buddha was King Makhadeva in his past life, based on the Pali version?
Are there other such canonical stories of the Buddha's past lives in the Pali Sutta Pitaka, outside of Jataka?
Are there other such canonical stories of the Buddha's past lives in the Pali Sutta Pitaka, outside of Jataka?
There's a bit in DN 16 -- not detailed, only that he was reborn here from a previous existence in Tusita heaven (the which event is one of the reasons why a great earthquake can happen):

Furthermore, when the being intent on awakening passes away from the group of Joyful Gods, he’s conceived in his mother’s belly, mindful and aware. Then the earth shakes and rocks and trembles.
Puna caparaṃ, ānanda, yadā bodhisatto tusitakāyā cavitvā sato sampajāno mātukucchiṃ okkamati, tadāyaṃ pathavī kampati saṅkampati sampakampati sampavedhati.


There's also the Buddhavamsa and the Cariyapitaka -- I'm not sure whether you count these as &quot;suttas&quot;: i.e. they're in the Khuddaka Nikaya (in the Sutta Pitaka), as are the Jataka tales.