Flame to high

SOURCE: my 2520 water heater won't stay lit for just the shower all other

Replace thermocouple

SOURCE: Toyo Laser 73 Flame goes out

You might have carbon build up in the heat pot, causing it to create a chain of carbon on the fuel rod. If the rod can not sense a flame the heater will turn off. A quick temporary solution is to give the fromt of the neat pot at the top where the flame rod is a tap to see if it loosens it. Start the heater up and see if it runs any longer. If it does, you need to have your heater cleaned, or clean it yourself. You might have a warped burner ring also.

SOURCE: pilot lights but when ignition pops back up the flame goes out

will either be the unitrol control box or the thermocouple, when you push down you are holding two magnets together and waiting for a small charge to be created from the probe in the flame , if this is not working you have no charge and the magnets won;t stick to allow the valve to open, change the thermo and see if that works first, its way cheaper than a new controler

SOURCE: Rinnai heater 403FT. The heater

I suggest you call for service. This "bang" could be a serious problem in the future - meaning the next time it tries to start.
Gary