Most likely, you forgot to keyframe those two individual bones. Make sure they are selected, then add the keyframe when they are in the position you want them in.
If they are already keyframed earlier in the timeline, in a different position, when you move your timeline cursor they will jump back to the position they were in at the earlier keyframe.
Make sure you are on the frame in which you would like to place the key, then do the bone movement, then place a key before changing frames.
Perhaps you would rather remove the original keyframe instead. This will avoid the jumping, and the bones will stay in the position you left them.
Another possibility is that you accidentally added a keyframe later in the animation, causing this problem. Maybe there is one on the frame next to the one you are on?
I'm sorry if this doesn't answer your question, but I do not have any other reasons why this problem is occurring from your post.