Update

I thought I'd found the solution last year as I gave up alcohol and it went away. However, I partied at Xmas and it returned but this time, giving up didn't fix it.

I'm having trouble finding any pattern to it but I seem to have some success with taking bitters prior to eating and bedtime to increase stomach acid. The one I use is A.Vogel Centaurium - it's foul when you start with it but you get used to it and it helps to throw it to the back of your throat. Seems more natural to me than taking ACV or Betaine HCL.

I've recently come across Norm Robillard's theory that the route problem could be intestinal gas (SIBO) and I'm starting his Fast Track diet that limits fermentable carbs via an FP value (Fermentation Potential). The app has a lot more FP values than the book and is reasonably priced.
https://digestivehealthinstitute.org/fast-tract-diet/

I came across his work via some old blog posts from Dr. Eades:
www.proteinpower.com/gerd-treat-low-high-carb-diet/
www.proteinpower.com/gerd-treatment-nutrition-vs-drugs-3/