Turns off every day, reset power and works fine

MAke sure your flu is not making any 90degree turns. Use 45degree turns only. Error 99 is a combustion error. Usually referring to a blocked/obstructed flu. That said, a flu wit a lot of 90degree turns can act like restrictions to tankless heaters. If the flu i straight and clear, then you have a burner problem. Somethign you want Noritz to deal with.

Have you cleaned the air intake screen located below combustion chamber?
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

http://www.thetankatwaterheaterrescue.com/forums/forum3/

Gene

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

maybe replace intellivent gas control valve:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

gene

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

Need a new thermostat to keep the unit from tripping

clean out gas chamber with vacuum. Also probably motherboard and rheem will send you another one.

Turn on the boiler and jump out the pressure by putting a wire between the 2 electric connections only after the unit fires up than put the jumper on, if it runs ok it is the pressure switch.

turn the water flow off and control off and hold the two arrow buttons together
then use the up arrow to get to e and the down arrow to get to 1 what do you see?

yep, got the same problem, try unpluging the unit for five minutes and the reset the temp.

Th epproblem you are most likely have is from a safety called a flag switch usually located either in the blower or right past it. When you unplug and replug you reset the switch. Sequence of events.  1 fan comes on. 2 switch is made from the airflow of the fan.  3 ignitor come on 4. main gas valve opens and ignitor starts flame. 
You are being stopped at step 2 when the fan does not move with the air. This shuts off and out the water heater from calling again.  Find the fan switch and make sure it moves easily. They get dirty and also dry from lack of lube. Good luck.  

Call the manufacture at 800-432-8373. They will be able to assist you. Have your model & serial number. Parts may be under warranty.