Last night before going to bed, I had a handful more then 230 adults, plus the rest.
Now upon loading, I have 30 less, then it continues to go down; after an hour of max speed play, my population is only 140! :-(

This is contrary to normal game flow, where they usually die off very slowly.

This is quite annoying, taking away from all my worker places after I worked hard to get them to perfectly balanced levels!

It's unfortunately not a bug. You've just hit a cycle where a large portion of your older population starts to pass away around the same time. Once you get your population in the 1000+ range the swings are really significant (+ or - 400 sometimes). The only way to battle it is by having enough children to replace the older population as they pass. The first time you see it happening it is very disconcerting.

If you had paused the game before your population had started to drop and had looked at the ages of your population you would have noticed quite a few in the 65+ range, and they don't live too much past that.

That should not happen!
5% up or down, sure.... never around half the population in the course of a few years!

As time goes on, the variance should become less and less because of the spread of age groups.

There are numerous factors in to how this can happen. Have you built schools and herbalists? Have you been building houses all at once or spreading them out every few years?

230 is a very small population. If you have not been building in a way that allows for continual growth (that's why you need to spread out how often you build houses) you will eventually die off. As I said earlier, if you had checked the ages of your population before it started dropping you would have probably noticed that the majority was older. I have been there, it's not a bug. This game is realistic. Your people live in the wilderness surviving on what they make or trade for and when and what you build. You will notice big drops in population if you don't find a way to keep your growth up. You said as time goes on the variance should be less and less, but only if you are building properly. If you build the majority of your houses within 10 years of each other then expect your population to drop a lot within a short time. If you are not keeping the health of your population up or keeping a big enough variety of food on hand, you'll notice a large drop in population at one time. Get scarlet fever and not have enough doctors or herbs on hand then you can expect a large drop in population at one time.

I have been building houses on a fairly regular basis; say every five years to ten years.
They have max herbs all the time and hospital, woodcutter, everything.
I couldn't find a tool that told me the ages, just the job spreads, so I could not see them without checking every house. >.>