Try booting in the Bios & tweaking the Power options under Power Management.
I think you need to set some value in the motherboard state to S3.
Have you seen if Dell is offering a BIOS upgrade for your Model. Issues which you are mentioning are typical related to BIOS settings.
You can try typing sfc /scannow at the Run Prompt of Windows. (Windows + R key combo will bring up the Rund Dialog Box). This will scan for & replace corrupt system files.
Ravi.