Ubt. 9.04 hangs when idle for >30mins
I've been experiencing a strange problem with Ubuntu 9.04 on an IBM R31 ThinkPad. When I leave the machine for about more than half an hour, it just hangs. LCD screen is off and no keyboard input is possible. No CTRL-ALT-DEL to restart and I have to shut it down by pressing the power button for 10 seconds.
I had no such a problem with Ubuntu 8.x on the same laptop.
What should I do now?