Peace treaties need to include a 10-20 turn NAP.
In my current game Tarth keeps declaring war on me, sending weak armies into my borders to just stand there and do nothing (my defenses are likely way to0 strong), when I kill a couple of them she asks for peace, then 1-3 turns later she's all "Your actions speak louder than words. Prepare to die" and declares war again.
Edit: Make that 0-3 turns. She just asked for peace, I accepted, and as soon as that window closed another window opened which was her declaring war. So I went from war, to peace, back to war, all within the same AI phase of a turn.