I've just tried it. I used both together for one character, and just the IK for the other character. Worked like a charm. I guess I was trying to move the character too quickly and too radically, which hborre cautions against above. Also, sometimes I tried to pull the limb I put the IK on. I guess I should move any part BUT the limb the IK is on, huh?

Well, thank you all very much for your help. I'm getting new enjoyment out of Poser.