Bosch AquaStar 125b ng suddenly goes cold during shower

your not making sense with you question --state it goes cold in middle of shower --but state it will stay hot or cold forever ----if its the latter continued hot till you shut off water that normal working if the unit shuts down during shower then you have a problem ----unit is set at one temp and runs at that temp and you use shower 1 handle / 2 valve what ever you own to adjust temp their are really only 4 reasons stops making hot water ----2 are usually well water supply --incoming water pressure /flow or to cold ----other are gas valve -or sensor so i would look into your shower handles since almost all have adjustments and anti scold safety

Check for crossover.
Bad single-handle faucet cartridge causes cold water to enter hot pipe, so hot never arrives and tankless does not run consistAntly
http://waterheatertimer.org/Crossover.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) If tankless is not showing error code, the problem might be single-handled faucet cartridge:
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

2) Catch up on maintenance, such as cleaning combustion parts, have gas pressure checked, etc.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Did you check to make sure the shut off handle is on the on position?

This should run two showers at once with no problem. If you have a shut off valve on the cold side of the heater, try closing about halfway. This will slow the water flow and the water will have more time to heat up as it pases through the tank. If you dont have a shut off valve, just cut back the hot water valve on your shower and do not use any cold water. If it gets too hot then turn on a little cold water. The object is to slow the speed of the water going through the tank. Tankless heater take a little getting use to but they work great when you know how to treat them. On my shower I turn the hot all the way on until it gets hot and then add a little cold water to bring the temp down. You may have too much water going through too fast to be able to heat it up to what you need. Try this and see how it works.

It sounds like you have a well. Make sure the water pressure does not dip below 30PSI. Otherwise, the gas pipe or the CO2 levels need to be adjusted by a qualified gas technician.

sounds like you may have "air" in the pipes and that bang has slamed the water reg valve shut hard

Sounds like hard water. Service the water valve. Parts are available from your local Bosch Service Provider (Contact Bosch if you don't know who the service provider is).

Take the thermocouple out and match it up at any hard wear store. Maybe $10. Replace it .Make sure the flame is clean and hitting the rod in the center center. Ajust (Bend) accordingly.

Replace the thermocouple.

Thats the flame sensor.

If the gas valve is not sensing flame it will not stay lit.

T-couple's are about 9 bucks.