macbook air wont turn on

Purchased last May (2014), just over one year warranty (or course). No damage, well looked after, so prior concerns/viruses etc.

Two weeks ago first problem: randomly turned off when in sleep mode. Couldn't turn it back on, just a blank screen. Tried to reset the the SMC and PRAM, when it randomly turned back on. Was fine until last week when this happened again.

At this point the SMC and PRAM rest were not doing a thing. If I listen carefully can hear a faint 'clicking' type noise when trying to turn it on. No lights, nothing, completely black screen. Green light on charger. Randomly booted up and sounded like the fan was overheating, was worried it would blow up completely and screen came on for a few seconds before becoming frozen and pixelated. At this point I closed the screen to prevent it from making the overworked sound and making the problem worse. Took it to authorised service repair place. And of course as soon as i got it there it turned on fine for them, leaving me looking silly. They ran a basic diagnostic and found no problems. Got it home and wouldn't turn on. Tried everything person on apple support line told me to try with no luck. Took it back the next day as wouldn't turn on at all overnight despite leaving it plugged into charger which was green. Got a call from the, today (4 days later) said it was sitting on shelf and they didnt get to look at it immediately and when the did it turned on as normal. They still cant find whats wrong.

Im hoping someone will shed some light on this issue, very frustrating as I need it for work/important deadlines.

Thanks.