OK so I'm writing this on my brand new 2014 Macbook air 11" which now seems to be stable. I was getting Kernel_task cpu usage ALL DAY yesterday of around 300%... The mac was virtually unusable. Fan was noisy, screen was jittery and the OS was unresponsive. I tried shutting it down, NVRAM reset and safe mode, none of it worked. Last night I shut it down via the log in screen, went to sleep, and today it would not turn on (magsafe charger not connected). Plugged in the charger for five minutes, pressed the power button and now all seems to be fine! Screen refresh rate is smooth, apps are responsive, and no crazy fan noise. I am currently powering it off the fully charged battery and OSX is claiming I have 11hr 55 mins remaining with screen brightness at 2/3, no music play and only Safari running. All seems fine (is that battery estimation alarmingly high though?). What worries me is that there might be some fault in the hardware that could rear its ugly head in the future... Is this likely? Has this happened to any of you? Should I return it for a new one even though it seems to be working fine now? Thanks for reading.