Try looking at the performance monitor and the logs that the system creates. Usually the system tells you when something is off or broken but many people don't look at the right places (and Windows doesn't really hint at the right places where to look).
I had the performance monitor up while testing, everything seemed fine, then it suddenly switches off. I didn't find any logs or any hints that anything was wrong. I don't know where to look either.
It's already done reverting, that went fast! The upgrade took hours lol. (probably due to my internet connection) All seems fine.
Are you still interested where you can find logs? I can only tell you where to find it in Windows 7 (and probably 8.1) :)