Inspiron Sleep Issue

I have a new Inspiron 15 (3521) running Windows 8.

I'm using it as a desktop replacement, so it's got an external keyboard/mouse/monitor plugged in, it's plugged into AC power, and I keep the lid closed.

I have the Windows power management set up so that if the laptop is on AC power, it should never go to sleep. I also have it so that if the lid is closed, nothing should happens.

So here's the problem: If I manually tell Windows to sleep, after I wake it up again, it'll go back to sleep on its own after 2 minutes of no activity. It'll keep doing that, until I open the lid and close it again, and then there won't be any problem until the next time I manually put Windows to sleep again. I can reproduce the problem every time. I'm not sure why it does this, or why it chooses 2 minutes of idle time before sleeping.

I've got the most recent BIOS and all the latest drivers from Dell.

Is this a known bug? Or is there something I'm missing? I couldn't find anything else in the Windows power management control panel, nor in the Intel graphics settings utility.