切换用户后黑屏

Question

以下是我在 Ubuntu 14.04 中发生的详细信息：

I log in as user "A"
Without terminating the session, I switch to user "B"
without terminating the session, i switch back to user "A" and there is a black screen in my monitor for a 3 seconds before start the session. 

你能帮我一下吗？谢谢