Keyboard & Trackpad not working - Macbook Air

Hi all,

Strange one, for a customer.

Macbook Air, intermittent Keyboard and Trackpad input. Tried SMC, NVRAM, PRAM resets, full wipe of data and complete fresh install of Yosemite, still nothing. Only works rarely (however power button works just fine).

Of note though is the keyboard will work for performing resets, and both keyboard and trackpad work fine when booting from an external USB (to choose boot option etc), but after this point they die again. Tested all the keys at this point too, all keys work. It's as soon as the Mac is past a certain point during boot that it seems to have this issue.

So very strange. Anyone any ideas what could be the cause?

Thanks so much.