Have a brand new Air. Some of the skips, or hops in playback seem to be related to processing on my iMac. Meaning, I can go back over the same section and the skip/hop may not appear. However, there are some that remain in the same location, as if the file was recorded that way. I'm using the recommended SanDisk Extreme Pro, rated for 100mps. I've only tried 4K @30 or 24    Anyone else have advise on this? thanks

Are you playing back from the SD card directly (via a caddy) or from a copied file on your main drive/SSD? If the former then there could be a transcoding issues from FAT64 (PC format). I'd suggest copying the video files to a new folder on your iMac hard drive - or at lease a USB 3 peripheral drive.

Sounds like a problem with the video player or read speed.

Possibly a slow SD card.. dropping frames during writes.

Unless the SD card is faulty, this problem will not be a read/write issue with the card. However, as has been rightly mentioned, it is always best to have the video file running from the computer hard drive.
Between the SD card and the processing circuit in the computer are many components that act as a choke to the flow of data. 4K at 60mb/sec is a lot of data to pass through.
I also discovered early on that a USB 3 hard drive is not a good device for sustained 4K throughput, even on a fast PC. Just because it says it is super fast, doesn't mean it is with sustained throughput, particularly if the computer is also reading/writing/processing something else at the same time.
There are also big differences with video players. At one time, VLC was the best for my setup, but now Win 10s video player works better for me.
On Win 10, SD card & external hard drive throughput to the computer is shown on a graph. It starts off with an impressive surge, then drops back to a slightly wavy line. Control-Alt-Delete will bring up the task manager and under the performance tab, you can see the data movement speed.

at first it sounds to me also like a possible media player settings issue. perhaps try to establish that first. start by trying another media player or try viewing your sd footage on another computer. if that works then your media player settings are out and can be corrected either by using another default media player or asking you tube about settings for your preferred player.
Also - it is possible that you have a faulty sd card - that happens. if your sd is not playing on other media players or other computers then you may want to try another sd card.

I've been playing back the files, thru QT Player, after copied to my i7 iMac SS drive.  I've been recording to a SanDisk Extreme Plus, rated for 100MB/s - 4K UHD.  I tried VLC, and have very little of the problems with 4K thru QT Player. QT Player will play 2K stuff fine, but it has a serious problem with 4K. There's some slight jittering thru VLC if I'm yaw'ing, but unless you can go super slow, a shot like that is terrible. Can't see anything as it pans around.  So..possibly, crisis has been averted and could be chalked up to QT Player not handling 4K. At least that's how it looks at the moment.

Try using 5KPlayer vice QT.