I've been running El Capitan beta 4 since it came out a week or so ago without any apparent problems. Then, tonight I was in "Mail" and the program started minimizing and maximizing repeatedly the Mail program VERY quickly. It was happening so fast I couldn't shut it down. Finally I was able to shut the computer down by holding the power key down. On the first reboot, it immediately started doing the same thing. On the second reboot, it seems to be fine although I haven't opened up mail yet. After I finish posting this, I will open Mail up again and will edit this thread if it starts again. Well, I just started up Mail and it's doing the same thing. This time I was able to access "Force quit" and it did close the Mail program, making Mail useless. I'm wondering if I were to try to REINSTALL beta 4 over beta 4, would this maybe fix this "flashing" up and down Mail program OR would it BRICK my MBA. I'm not a happy camper at the moment. Wonder of wonders...With the Mail program shut down, this time I used the "Restart" option under the Apple in the upper left hand corner and the MBA rebooted. I then opened Mail and the rapid flashing minimizing and maximizing stopped.