yeah, networkmanager was giving us too much headache and we turned it off.
if you just enabled it one time, then rebooting should help.
if you enabled it permanently via chkconfig, you should probably consider undoing that.
(if you don't know what i'm talking about, then you proably did not enable it permanently)
you could also try:
however, note that that command above will reset your network settings, including any new settings you may have added recently (aliases, static IPs, etc.).