Furnace Issue

So I'm at the end of my rope. Atwood 8531-iv furnace in a 2005 Max Lite. Furnace started intermittently running ok and then not running at all. When it didn't run, it would start the fan, ignite the burner, run and pump hot air 5 or 6 seconds, then lose flame and retry two more times doing the same thing and lockout throwing an ignition fault code.Figured it was probably the flame sensor so I pulled the unit and replaced the ignitor/flame sensor and put a new burner in since I had it out. Ran good for one night, cycling as it should then started doing it's old tricks again. If I understand how this works correctly, since it's starting up and burning 5 or 6 seconds, it shouldn't be the sail switch or control board. Am I correct in this assumption and if so does anybody have a clue what else it could be? Also should mention I have a brand new thermostat I just put on.Thanks in advance for any help!