The last two forts I've built (0.31.18 & 0.31.21) have become very mature, healthy forts but ultimately both hit the same problem- they'd suddenly be too slow to be playable.
I know that as a fortress gets bigger (more dwarves, more animals, more items, more mined/discovered tiles) things slow down- that's normal. I also experience frame rate tanking out when bodies of water freeze/thaw and that sort of major event- typical calculation/processor load stuff.
But in both of these games, things went from regular slow-ish-but-playable speeds to suddenly stop-and-go choking speed/stalling/hanging. It gets totally unplayable. And the weirdest thing is, it happens suddenly and I can't figure out what's triggering it.
Sometimes I can fix the slow down by reverting to an old save and the game might play fine for a while- even past the time when it slowed down before- but eventually they all come to a choppy halt.
The 0.31.21 fort was about 10 years old before it suddenly got too choppy to play. It was shortly after a goblin siege but no other major events/season change that it got all choppy. I'd recently reduced the stockpiles a lot by dumping items in magma so I don't think overstocking was the reason it slowed down.
Also it should be noted these are the only two forts I've played on this computer- Windows 7, Intel i3, 3Gb ram- maybe it's the system that contributes to the instability?