What size are the stills (mb ?) and what format are they?
What preset are you using? ( BM?)
Each still is a 6MB BMP. The preset is
Blackmagic Design/HD1080/8 bit YUV/PsF/30p.
At the risk of sounding foolish I've had extensive problems with stills over the years.
I almost always solve them by bring them into Photoshop and making sure they're of the right size for the project I'm working on, and in the RGB colourspace. Sometimes I leave as PSD's, sometimes push them out as high quality JPEG's.
Give that a try.
You might have much better luck, and also higher quality, if you resize your images in PS, prior to Import. This ARTICLE will give you tips on automating the process. Also, the quality will likely be a bit better.
Maybe it's because I go from camera RAW to .PSD, then resize to the Project, that I keep the images in .PSD. I have not been a fan of JPEG compression, going back to the earliest days. I only used it, when a client needed images for their Web site. I'm sure that there are good reasons to use it, especially with the higher quality/lower compression settings, but I'd just as soon keep my workflow in .PSD.
I would suggest a quick test to see if the problem is 3rd party related.
Setup a native Pr project rather than a black magic preset. Use adobe transitions rather than Hollywood fx. If it works correctly, then we know to trobleshoot the 3rd party stuff; or not.