Computer doesn't return to sleep mode anymore

Greetings,

My computer is a new Alienware Aurora running windows 7 64 bit Ultimate version; it also has an internal Hauppauge WinTV HVR 2250 TV tuner installed.

I've occasionally used the built-in Windows Media Center to record TV programs, this was how my computer used to behave:
1. I set time for TV recording,
2. I put computer into sleep mode,
3. My computer woke up at the appropriate time,
4. did the recording,
5. put itself back to sleep afterwards.

It's been a while since I did the last recording, I just found out recently that my computer now behaves differently: it only does step 1 to 4, it doesn't put itself back to sleep anymore. I came home a few time to find the computer was still on long after having recorded the shows, when it should've been in the sleep mode.

I'm scratching my head over this sudden change of behavior. I didn't change any power settings at all. The only hardware change has been the 850w PSU, when the last PSU failed and had to be replaced under warranty service.

TIA.

Thank you for the reply.

I read the thread you posted, unfortunately didn't find anything relevent to my particular program.

Perhaps I can rephrase my question from a different angle. When my last 850w PSU failed, Dell/Alienware sent out a technician and replaced it with another 850w PSU, though I can't be totally certain the replacement is 100% identical. I wonder if it is possible at all, as far as you're aware, that having a slightly different PSU could cause a computer to fail to return to sleep mode properly.

TIA.