FCP X hang on background render

Using FCPX 10.2.1 on OSX 10.10.4 on a MacPro 2 x3.06 GHZ6-Core with 64GB of RAM.

FCPX began freezing on playback of projects with very simple elements, as in audio vo track and a couple of clips laid over with dissolves. Clips are ProRes 422 HQ. This would require a forced restart to continue.

Projects would play a few times and then on playback the image would freeze in the monitor as the project continued to play.

I trashed preferences, I trashed renders, I tried a different library, I created a new library with the same clips and new projects. The issues persisted.

I began to look at the background render display. While idle the renders would start and then hang at somewhere around 50%. If I forced render, the renders would complete (actually, they would start over).

This situation continued to degrade and now I can't even force a render on dissolve on one clip to black. At this point if I move the skimmer to anywhere on the timeline I get the spinning beachball and have to force restart.

Any suggestions?

Thanks

Bill Buchanan