I've had the same 99% to 50% drop problem as everyone else.

I changed one prefrence. I disabled the Drop and Retry Incorrect
Downloaded parts in the Download tab under Prefrences.

The movies that doprre and restarted now merge niceley and play all the way through. I don't know if this will work with two drastically different files, but it has worked on three movies I've spent a week trying to load.

hope this helps,