I'm trying latest build of AVG 2014 IS(earlier build I tested was a little bit buggy on Win8.1) and so far its been fine so looks like this latest build has fixed the issues I had ie speed improvements,odd pause gone.
Avast 2014 RC3 is also another favourite of mine too.
Update to my previous post above,AVG 2014 IS is still working great on Win8.1 and it has given me no issues,so very positive so far,lightweight and no slow downs while gaming/browsing etc...