Hmm so if you are working on a previous project that was fine on the older MAC it does seem unlikely that anything has changed in terms of the media.
I suppose it could potentially be a monitoring thing although I would have thought it was unlikely. Does it always occur in the same place or is it random ?
Have you any other projects you can open and see if it happens there ? Does it only happen on this sequence or do you have any other sequences you can check ?
If you export a section that is displaying the flickering does it show up on the exported file or is that OK ?
Kinda hard to diagnose without seeing it really.
Maybe worth doing the usual prefs trash and permissions repair just to eliminate the basics.
https://www.digitalrebellion.com/prefman/ – this is the easiest way to trash prefs.
best wishes
Neil
http://www.patience.tv