So because I have a tankless ... does make me a republican, democrate, independent, communist ... no need to reply :D
You didn't mention why the previous units needed to be replaced. As far as the current unit that you're having issues with, it sounds like the burner may be dirty and the flame is not contacting the flame sensor. It also sounds like the T&P valve is limed up and needs to be replaced. It would be worthwhile to completely clean the burner (rod out all the openings and vacuuum out). Have a plumber verify that the the fuel to air ratio is correct as well as the gas pressure at the valve.