Thanks for the response. Restarted and haven't seen the problem again. Maybe just a glitch. From what you say, it sounds like possibly an uninitialized pointer. I've done that before :-)
My CPU rarely gets above 25% with everything I have running on three screens. It's an I7-3770 running at 3.4ghz with 32 gig of ram. Drive is a fast SSD. Don't think it is a load problem.
I really enjoy the program. Thanks for writing it! I'll pay more attention if it happens again and hopefully provide you more info including cpu load.