This is one of those incredibly difficult to answer questions, because it's like asking why anything else disappears, and how to stop things from disappearing. The task bar process might hang, or be terminated, by something. Kdeinit might die, or the DCOP server may go defunct once in a while.
I would try an update:
linux # sudo yum update
If the hardware or Internet connection is slow, you might selectively update, using:
linux # sudo synaptic
In which case you might need to:
linux # sudo yum install synaptic
If you aren't in the sudoers file:
linux # su
This is the vi editor.
Press the insert key.
Scroll down with the down arrow key.
root ALL=(ALL) ALL
And put underneath it:
<username> ALL=(ALL) ALL
Where username is your username.
Hit the escape key,
':x' and Enter key.
You are now a sudoer, so you can sudo.