Moving this to ePO.
By 'disappears' do you mean the agent icon no longer shows up in the system tray? Please check if the UdaterUI.exe and McTray.exe processes are still running and not actually quitting for some reason.
Check the application event log for any errors...
btw, what version of the agent?
What if you do a command line option to force the status monitor to come up? < \your system directory\mcafee\common framework\cmdagent.exe /s >
That should bring up an agent monitor. If it does not, then it seems like your installs are being corrupted or overrode by another service/process. Let us know...