My Rheem Criterion model has stumped repair man. We have replaced the sensor, the gas valve and changed out a computer board. The heater will fire up some of the time. Most of the time it will go through the cycle of ignitor glow, burner kick on for a few seconds and go out, the blower works fine. Any suggestions?

Sounds like you have a venting issue. Something has to sense that there is air flow in your vent. If it does not sense sufficient air flow in the exhaust, a relay will close the gas valve and shut off the burner. I had a vacuum switch on a boiler that gave me fits... same thing as you describe. I don't know what controls you have but I would check your vent.