Macbook Air mid 2013 on Yosemite hangs
sry cant comment (reputation)
i have an older Macbook Air mid 2009 13" with 2GB of RAM and SSD, but found that many user experience similiar bugs as i do on Yosemite:
https://discussions.apple.com/thread/6624349
(some on macbooks, some on iMac's)

it seems like i solved mine (for now, had no problems for the whole day) by resetting PRAM/NVRAM multiple times in a row. Im still not sure which caused the problem. Alternatively if i will go back to Mavericks or Mountain Lion. Yosemite feels to me like Windows Vista (altough i like the design)