Strange animation bug

I have a strange animation problem where the viewport, while browsing frames, will have bones partially updated in their positions, and I could only see the full frame if I switch frames to the next then previous. This affects exporting.

Using Blender 2.66a (and no later due to some regressions)

I’ll upload a blend file later with a generic man mesh with the problem, since I would have a problem sharing the current mesh for license reasons.

(Had to write this post again but much shorter)

Strangely the bug went away when I made a new action. However I think my attempt at an ik rig is suspect. Here’s a video