Macbook Pro requires charger to be awoken when held sideways.

I have a mid-2012 Macbook Pro 15" with 2.6 GHz Intel Core i7, 8 GB 1600 MHz DDR3 version 10.8.2. This problem only occurred to me quite recently, but often.

When my Macbook Pro sleeps I can usually awake it by tapping the spacebar. In this case the white light stripe is acting as it should. But when I pick up my sleeping Macbook Pro and hold it sideways (something I have done often, to take it with me), the light suddenly turns off, as if my macbook were turned off. I then cannot wake it by tapping the spacebar or even the power button. When I connect my charger, the white light turns back on and I can wake it as before.

This happens even when my battery is above 90%. My Macbook doesn't say my battery is broken.

I read somewhere I could try and reset my PRAM, I did this but it didn't help.