Nothing more frustrating than this.
Hope you can keep it together to figure this out.
Take Robs advise, because it really helps.
cut down on the timer, try less run, try more time in between cycles.
But the high limit can be controlled, mine doesnt go above 180 now, but I have my limit set to 160-200 or so.
With DHW demand and timer cycles, it stays around 160-170 most of the time.
Just hang in there with more fiddling.
Keep record of coal purchase, temps and settings.
Again, just getting a few bags of another brand, you could put in your barrel and see if that helps for the rest of the summer,
but chances are it is just a matter of settings...... on the air/feed and timer cycles
Give updates if you can, we are all here to help.