I have the exact same issue @ UE 4.14.3.
ALT+Tabbing around when Unreal is open leads to random permanent freezes which totally shut down the editor instance. Most often (if not always) the Unreal Process seems to be stuck in a loop, because task manager always shows it at 15%+ cpu load while its frozen.
Only solution is to kill it from taskmgr and restart.
I do encounter these freezes multiple (5+) times a day, if I work with different programs and switch windows more often. It’s not only a huge annoyance, but also a massive slowdown of every task in Unreal Editor… this issue has to be fixed asap.