After 10 years of perfect operation this exact same problem suddenly appeared with our minimalist Bosch Aquastar 125B (no remote and about 117,000 BTU/h burner). It took a while to figure out the erratic operation was due to a new lower flow rate shower head I had installed first in one shower used mainly by my wife. I ended up removing the plastic disc flow restrictors in the handles of both of our two shower heads and the problem disappeared. The heater was shutting off due to flow dropping below the minumum.
Since we are responsible adults we can control the flow using the valve settings and don't use an outrageous amount of water. If you have users who can't or won't do this, you may be able to modify the flow restrictors or get new restrictors that allow say 2.0 gal/min flow rate.
For a while I had my wife run a small flow through the hot tap of the lavatory, but this seemed a waste and not reproducible so I fixed the head to allow a greater flow rate.
Background: When we got the tankless heater we had standard 2.5 gal/min shower heads and the tankless heater was not erratic. My wife later installed a 2.5 gal/min hand held head in the shower she uses. After some years of use it conked out and I replaced it with one with 1.4 gal/min flow restrictor and didn't realize that this would cause her erratic operation. I first took out the flow restrictor in her shower and drilled out the orifices a little, but still it would not keep the heater on reliably. I could have drilled more and tried again, but I just kept the restrictor out.
I tried to get new 2.0 gal/min restrictors to fit our shower heads, but couldn't find any locally that fit the small diameter of our very basic shower heads so I have removed the restrictors from both shower heads. These are $15 ea plastic heads with flexible hose so one can use in the hand. Great for washing feet and the dog.