I'm quite embarrassed too. The random artefacts in the H.264 files are extreme visible in CC 2014 (to be honest in previous versions I haven't seen this). Adobe offers complex integrated workflows and it cannot be that the suggested solution is a downgrade or even using another tool --> the whole workflow (Prelude, Premiere, Speedgrade, After Effects, Audition, Adobe Media encoder) is broken and adds hours of useless goofing with workarounds! This is the most serious topic since I work with Premiere (8 years+ I never ever had a customer visible malfunction of PP) and I still don't feel the matter of urgency at Adobe!
PS: BTW I'm on an iMac late 2012 32 GB memory, 2G GPU and 32 TB Raid6 storage and the H.264 materials come from several Sony CX7xx cameras