Lock screen allows input to running applications

[sonicsteve]

When I lock the screen (CTRL-ALT-L) or if the screen locks itself due to inactivity, I can often see through to the desktop programs that are open and use them for a short time before the lock screen takes over again and prompts for password. It usually only lasts for a few seconds.

For example, lock the screen, move the mouse, the screen wakes up and I can see thunderbird or firefox and actually use them for a moment or two, then the screen will lock again prompting for a password.

Does anyone know where I should start with this problem?

I'm using intel based graphics if that makes any difference.

Thanks for any help,

Ubuntu 12.04 64bit,
fully updated. Using security, recommended and backports, not proposed
List of my devices;

00:00.0 Host bridge: Intel Corporation 2nd Generation Core Processor Family DRAM Controller (rev 09)
00:01.0 PCI bridge: Intel Corporation Xeon E3-1200/2nd Generation Core Processor Family PCI Express Root Port (rev 09)
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)
00:16.0 Communication controller: Intel Corporation 6 Series/C200 Series Chipset Family MEI Controller #1 (rev 04)
00:1a.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced Host Controller #2 (rev 05)
00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High Definition Audio Controller (rev 05)
00:1c.0 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 1 (rev b5)
00:1c.4 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 5 (rev b5)
00:1c.5 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 6 (rev b5)
00:1c.6 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 7 (rev b5)
00:1c.7 PCI bridge: Intel Corporation 82801 PCI Bridge (rev b5)
00:1d.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced Host Controller #1 (rev 05)
00:1f.0 ISA bridge: Intel Corporation H67 Express Chipset Family LPC Controller (rev 05)
00:1f.2 IDE interface: Intel Corporation 6 Series/C200 Series Chipset Family 4 port SATA IDE Controller (rev 05)
00:1f.3 SMBus: Intel Corporation 6 Series/C200 Series Chipset Family SMBus Controller (rev 05)
00:1f.5 IDE interface: Intel Corporation 6 Series/C200 Series Chipset Family 2 port SATA IDE Controller (rev 05)
03:00.0 IDE interface: VIA Technologies, Inc. VT6415 PATA IDE Host Controller
04:00.0 USB controller: ASMedia Technology Inc. ASM1042 SuperSpeed USB Host Controller
05:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 06)
06:00.0 PCI bridge: ASMedia Technology Inc. ASM1083/1085 PCIe to PCI Bridge (rev 01)