Update*
Not doing a victory lap yet, but I finally got time and doubled-checked the ignition coils.
I didn’t notice the first time around, but the mounting holes on the back of the box where the coils mount, were totally corroded/rusted to the point that 2 were cracked/broken.
I ended up replacing the battery box with a clean used one off eBay. I also cleaned the coil arms that are held down to the mount holes with the bolt and tab.
After 2 longer test rides, the stalling/studder/powerloss symptom hasn’t come back.
I’ll post again after I’m fully sure that was what caused the problem.
Thanks for everyone’s help.