When I bought the game, I believe it was under a 9.x version. I do NOT use dev snapshots, I merely have the launcher, which feels obligated to download a jar each and every time I start it, and ask to run minecraft.

So, when I played, at first, it worked fine. Just fine. I dug nice tunnels, built nice things, etc. Despite having put the visibility limit to 40 or so chunks, it DID work (albeit slowly).

Now, I made a pause for a while, and, when I wanted to play, this time, 10.x was out. Great, polar stuff and the like, I was impatient to try. So, I tried to load my nice world, and I got a crash after it ran for, what, 3 seconds ?

The crash usually tells me:
[00:07:29] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2103ms behind, skipping 42 tick(s)

Of course, the actual values vary, but the message is there. Sometimes, too, I get a core dump (well, NO core dump, because they're disabled by default on 8.1, but it tells me an access violation occurred).

A few times, I managed to hit F3 and then ESC before the game crashed. When in the menu, I lowered the chunk display limit to 2, thinking the crashes might be linked to a huge area being loaded at once, at startup. But nope. I was seeing 0 chunk updates, while my memory usage kept increasing, by about one MB at a time (on the upper right hand corner), all while being on pause.

When I unpaused the game, sure enough, it crashed after mere seconds.

The worst thing is that I cannot even use an old version to play, because those crash, too. So... did Mojang "update" those old versions with newer code ? Or should I look for another cause, like, maybe, a Java version update ?

I did NOT update anything on my own, but it is very possible that a Java auto-updater to have acted without telling me anything, for instance. Are there known incompatibilities with some JRE or JDK versions ?