Games Running Sluggishly After Waking from Sleep

After waking my computer from sleeping, games run at significantly lower frame rates. For example, Left 4 Dead 2 usually runs at over 100 frames per second for me, but after waking from sleep, it runs at 30 to 40 frames per second.

Have also noticed this in other Source games, such as Team Fortress 2, Counter-Strike: Source and the Half-Life games, as well as in Awesomenauts and Beat Hazard. This has always been the case with this graphics card.

Restarting the computer rectifies the problem.

Thank you.

System specifications:
Ubuntu 12.04.4, 64-bit
Unity Desktop Environment (vanilla)
Asus M4A78LT-M LE Motherboard
Asus BIOS Version 0803
AMD Athlon II X4 645, 3.10 GHz
8GB RAM
Nvidia GeForce GT 240
Nvidia Proprietary Graphics Driver (Version 331.38 Post-Release Updates)