You know that the Plackback optimisation is just that, for smooth Playback in the editor? You don't need it to complete to export the file.
You may not even need it active if you have a powerful computer or working in SD. Some setups will only need it at 50% or so to get the transitions and effects to render.
There are a number of things that can stall the PO. If it stalls on a particular clip every time, is there an effect on that
clip that might be the cause (some of the older effect can do this in
particular)
If you have the PiP or Crop control activated, PO rendering pauses.
If you have a stablise effect that hasn't completed the analyze phase it will be working in the background. A nested project may also be doing that - it may be rendering but there is no sign that it is.
Open the task manage and see if there is any significant CPU usage when you think it has locked up.