Ubuntu desktop blackscreen

I run Ubuntu 11.04 on a Dell OptiPlex 755 workstation.

My problem is that occasionally and for no apparent reason my screen will not redraw after it has been locked or the screensaver has kicked in.

I do not see the unlock dialogue, but I do see the mouse cursor, however nothing else. I can switch to any of the other console
terminals using ctrl-alt-f[1-6] and do see the login prompt. My system does not hang, but the GUI session simply blanks and I am forced to restart my machine to resuse the graphical.

I've never had this problem before with previous versions of Ubuntu the last 3 years, only with 11.04.

I have the following hardware:

00:00.0 Host bridge: Intel Corporation 82Q35 Express DRAM Controller (rev 02)
00:01.0 PCI bridge: Intel Corporation 82Q35 Express PCI Express Root Port (rev 02)
00:02.0 VGA compatible controller: Intel Corporation 82Q35 Express Integrated Graphics Controller (rev 02)
00:02.1 Display controller: Intel Corporation 82Q35 Express Integrated Graphics Controller (rev 02)
00:03.0 Communication controller: Intel Corporation 82Q35 Express MEI Controller (rev 02)
00:03.2 IDE interface: Intel Corporation 82Q35 Express PT IDER Controller (rev 02)
00:03.3 Serial controller: Intel Corporation 82Q35 Express Serial KT Controller (rev 02)
00:19.0 Ethernet controller: Intel Corporation 82566DM-2 Gigabit Network Connection (rev 02)
00:1a.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4 (rev 02)
00:1a.1 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #5 (rev 02)
00:1a.7 USB Controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #2 (rev 02)
00:1b.0 Audio device: Intel Corporation 82801I (ICH9 Family) HD Audio Controller (rev 02)
00:1c.0 PCI bridge: Intel Corporation 82801I (ICH9 Family) PCI Express Port 1 (rev 02)
00:1d.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1 (rev 02)
00:1d.1 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2 (rev 02)
00:1d.2 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #3 (rev 02)
00:1d.7 USB Controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1 (rev 02)
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 92)
00:1f.0 ISA bridge: Intel Corporation 82801IO (ICH9DO) LPC Interface Controller (rev 02)
00:1f.2 IDE interface: Intel Corporation 82801IR/IO/IH (ICH9R/DO/DH) 4 port SATA IDE Controller (rev 02)
00:1f.3 SMBus: Intel Corporation 82801I (ICH9 Family) SMBus Controller (rev 02)
00:1f.5 IDE interface: Intel Corporation 82801I (ICH9 Family) 2 port SATA IDE Controller (rev 02)

July 6th update:

Well the screen problem has happened again. Between now and he last time, I tried changing screen savers to see if
this would resolve the issue. When I switch to ctrl-f1 screen, I killed the screen saver process, then went back to graphical but
there was no improvement. Although I could move the mouse cursor around the screen, there was no redraw. I tried
alt-tab to see if tabbing through running apps would force a redraw, I tried clicking on various areas on the screen and still no joy.

I've really no idea where to start t'shooting this matter as from a command line point of view, the system is 100% usable.
As I use this desktop for work and therefore depend on the graphical, I need a solution.

July 7th update:

After disabling entirely my screensaver (I think) yesterday, I arrived this morning to find the same problem, but slightly
different this time. Instead of a black screen, I had the full screen display of my last session before i went home for the evening.
The mouse worked, my synergy was still working as I was able to move the mouse to a different machine's screen but the
screen image, which displays my clock showed a time from several hours ago, presumably when things turned bad.

Normal behaviour would have me type my password to unlock the session as I did lock the session when I left for the evening.

I've still no idea where or what to t'shoot this matter. Log files show no obvious bloodstains and the matter remains a mystery.

July 13th update:

Well not sure if anyone cares about this as I've had absolutely zero feedback from anyone. But I think I have found a solution as it has been several days since I have had the annoying black screen problem stuffing up my GUI session.
I disabled the Power Management and so far so good. If anyone else is unfortunate to have same problem as me, do visit the following: System, Preferences, Screensaver, Power Management. Under the Display option, choose Never.

Works a charm for whatever hideous bug was lurking in PM for my hardware under Natty.

Good luck and goodbye.