DaVinci Resolve Post Editing

Hi and first off i am a complete newbie to DaVinci Resolve and mainly a photographer so this may be quite a basic question, i am using the Canon XF305 so the files are MXF which Resolve obviously has no problem importing my computer is an I Mac 27inch (approx 18 months old) the GPU is a NVIDIA GEForce GTX675MX 1024MB it now has been upgraded to 32gb memory ,my question is upon trying a few test clips from the Canon when i playback i sometimes get very bad jagged lines around the edges of objects especially whilst moving the camera side to side the camera is set to 1920X1080 50i the clips playback fine on the camera monitor i also notice when going into "clip attributes"in DaVinci Resolve and changing aspect ratio to PAL 16:9 all looks fine on the playback monitor so any guidance from the seasoned pros out there appreciated, in addition when using an external drive (scratch disk) are there minimum requirement to be able to use it for working from to avoid large projects filling up my hard drive on the Mac, many thanks for your patience reading this lengthy post.

Tony Swain