What exactly are you doing that is causing instability?
Okay after some really extensive research I have pretty much figured out how to do mystcraft but the instability things really suck so I will probably just go into the config and disable instability and world decay. I probably will also get into rftools dimensions because that looks like it might be more specific or at the very least a lot of fun.
So what, exactly, are you writing? Why do you have instability?
It is not hard to say "stone block, water block, flat world, mushroom biome, single biome distribution, normal lighting, full speed, normal sun", and get a basic world with minimal instability. Probably no instability, at least unless you get a random feature (note that I didn't specify ravines, caves, or mineshafts) that adds them. And if you do specify caves, ravines, mineshafts, and dungeons, you probably won't get any instability at all. Heck, add water block, surface lakes, lava block, underground lakes if you really want an almost clone.
If you want more ores than normal, then you will get instability unless you do some intelligent writing. Look into either deep ocean flat worlds, or island worlds, as ways to cut down on oregen to reduce the cost of worlds.
Oh yea -- If you want to abuse things, look into obelisks + island worlds.
That will give you mining ages, just not the sort you are used to.