Sleep function is not working properly

I own a Dell XPS M1530, and my sleep function stopped working properly.

My laptop used to be fine, but for the past month, whenever I try to put my computer to sleep ("clicking start + then sleep"; "Fn + F1"; "closing my laptop's lid") my laptop will basically restart instead of sleeping. Sometimes when my laptop restarts it will even say my laptop was shutdown improperly and it gives me the option of starting in safe mode or normal mode.

Has anybody else had this problem before and found a solution?

After doing some research, I found that there are many Dell users who have had this problem, but there are to clear solutions.

I have already updated my drivers, BIOS, and all of the Windows updates.

On this forum: http://forum.notebookreview.com/showthread.php?t=260432

some users said the problem could have originated from the NVIDIA graphics card. I updated the driver for my graphics card, but the problem still persists.

My dell wifi card is the only thing that I can think of that could be the problem.

Please help me solve this problem. IT IS VERY ANNOYING!!!