Acer Aspire X3810 - no power - sleep mode?

Hi,
Last week I bought an Acer Aspire X3810 (Intel E5200, 3Gb ram, 1TB harddrive, Vista Home Premium).

I have had a problem occur twice, a few days apart, where the following happens when I try to switch it on.

- I press the power button
- hard drive light flashes once, very briefly
- power light (on the tower) and caps lock (on the keyboard) flashes once, very briefly
- everything shuts down
- several seconds later (about 3 seconds) the power light comes on, stays on, and nothing else happens (nothing on the screen, no other lights), there is not even the usual beep to indicate that the POST is happening.

The first time it happened, I packed it up and drove back to the shop, only to find it working perfectly. Today, I left it unplugged for a while, then I opened the case to see if there were any loose connections (couldn't see anything wrong). When I plugged it back in, it worked fine.

Incidentally (coincidentally?), the machine was completely hanged before this problem - I had to do a hard power-down (press power for several seconds). It just totally froze up while I was listening to some music in Windows Mediaplayer - the display was still on but frozen, and the only thing it would respond to was the hard power-down. I can't recall whether the computer was switched off normally the first time or not.

I have Vista set to not use any sleep or hybrid sleep modes.

Does this sound like some sort of sleep mode is getting initiated, and leaving it unplugged is clearing some flag in the bios? Or does it sound more like a loose connection that is temporarily fixed by hauling the PC around? Or can anyone suggest anything else? (I am preparing to take it back to the shop for a refund).

Many thanks for your time (apologies for post in 'Dell' sub-category, there isn't one for Acer, and TomsHardware is the only tech forum I subscribe to).