I've seen this problem before. In my case it was due to a driver problem- the graphics card wasn't coming back online again after waking the PC from sleep.
Try upgrading your graphics card drivers and seeing if that fixes it. If you are determined to fix this- you'll have to find a driver (or a version of your existing driver) which handles power state change properly.
Failing that- try disabling sleep mode and just letting the screen power down after an hour or so.
I've seen far too many problems with S3 (sleep) and S5 (hibernate) to want to use them any more or to recommend them to others. I will happily use screen dimming/ screen power off, and hard drive spin down options- but I no longer let my systems go into different power states due to the risk of total session data loss due to problems going back to S0 (I.e. waking up again)
The other thing about keeping a system running in S0 (just with the screen off) is that it will be able to defrag the drive, index your files, and install updates during the idle time- so it will do something constructive while you are away. If you are going to be away for a long time and this won't be connected to AC, then shutting it down is your best bet anyway as far as battery life and reliability is concerned.