Macbook air locks up instead of standby. Cannot wake with screen black and no sound. Have to hold power button for total restart. Happened with Mountain Lion and Maverick

I've had this issue since first buying my MBA mid-2013. Instead of going to standby, which works 98% of the time, every now and again it appears to go into standby with the screen black, but no amount of key pressing or mouse moving will wake it up. I am forced to hold down power button for a manual restart and faced with losing work. This problem has occured whilst using Mountain Lion and still occuring since updating to Maverick.