iPad2 screen turns grey and then icons appear when using Safari

In order to run a specific app, my wife's iPad2 had to be upgraded from iOS7 to iOS9 (now iOS9.2.1). Since the upgrade, the screen often turns grey, when using Safari, and after a few seconds the desktop icons appear. This typically happens several times a day. It doesn't seem to be dependent on which website is being viewed at the time.

We visited the local Apple Genius Bar and the person there updated some apps and said that if the problem continued, the iPad2 should be restored to factory settings. As it did continue, I did the factory restore but this didn't cure the problem.

Any help to solve this problem would be appreciated.