Spark running app version 4.2.20 and firmware 01.00.0900, since updating to that revision (no idea what it was before, product doesn't actually have a good system management interface) - anyhow, since updating to that firmware my drone continually (every 30-60 seconds) flips between 2.4 and 5.8 frequences, temporarily loosing contact with the craft, then re-establishing.  This did not occur before I updated firmware this morning.

Anyone else having this issue?

Hi, We apologize for the inconvenience. Please try to restore the factory default and refresh the firmware using DJI Assistant 2. Please download the software on this link: https://www.dji.com/spark/info#downloads

I've not seen that behavior before.  Can you possibly do a screen recording on your phone when this is occurring and share that video out here?  Sometimes its useful to see the behavior firsthand rather than just a description of it.

A video review would be good to see