Proxy playback issues

I'm working on my new MacBook Pro (15 inch 2.9 GHz i9, 32gb ram Graphics Card: Radeon Pro Vega 20).  It's performed well for me so far, has handled 4k footage smoothly, with no issues.  Overall I've been pleased running Premiere on it.  But I'm currently working on a project shot on 4k  at 60fps (with the Sony cx 350).  I created h264 720p proxy files.  After interpreting the footage shot at 60p, assuming the frame rate is 23.98 -  It's playing back very choppy in my source monitor and in the timeline.  So much so it's almost impossible to edit.  I've checked that the proxies were properly attached and they are - not sure what is going on. I've used this same workflow with proxies and interpreting 60fps footage on much slower MacBooks and have never had this much trouble.  Would much appreciate any and all trouble-shooting suggestions!  Thank you.

-Bill