I've been working on a commission and encountered these same problems. I've dealt with them by not using IK and by splitting the animation into multiple files. (For example, if frames 200-210 make frames 190-199 go nuts, then I put 200-210 in a different file). I've found that also splitting up the animation makes for less strain on my computer during rendering (i.e., my computer only has to render all night instead of rendering all night and half the morning, until it locks up).

I agree wholeheartedly with TheOwl's analysis. The video I'm working on has turned out to be far harder to make than I thought it would be, but every subsequent animation I do is faster and easier because I remember what I learned during the last one.

~*I've made it my mission to build Cyberworld, one polygon at a time*~

Watch it happen at my technology blog, Building Cyberworld.