I did some memtests and sfc /scannow to check the system, no errors found. I've also been happy with the system before 8.1 update. Also, I reinstalled OS a couple weeks ago and everything was fine in Win8, and as soon as 8.1 was installed the problems started. So while nothing is conclusive... I'm pointing at Win8.1 being the culprit.
I'll try disabling Intel Graphics in Device Manager next time and do some more tests, unless you can suggest anything else. I'll report back with more minidumps if it's useful. Thanks.
Regarding the drivers:
Yes, that's what I downloaded. Says 14.4 on package, but 14.100 in brackets, so it must be the latest one indeed. The Win Update one went back a version actually.