Why is this oscillator unstable?
What's happening is that the simulation is not deterministic! What an exciting property for a simulator to have! It appears that if it takes too long for an event to occur (computationally) it gets delayed a time-step. If you set the simulation speed too fast, you may get a different result!

At any rate, your flip-flop is getting clocked twice at one of the peaks, and that causes the output to rail or do other silly stuff. 

You can improve this circuit by using a S-R latch instead of a flip-flop. That way, even if you have glitches out of your comparators, it's the first glitch that counts. Second, there are no possible invalid states. Notice in your image how if the Q_Bar output = 0 and the output wave is below -0.2V, your circuit is permanently stuck.

What you really want is something like a Set-Reset latch. That way you don't care about the comparators glitching, because your circuit will still function. Here's a link back to the Farstad simulator, as well as a screenshot.