Had similar issues.
In the end it was just two things:
Poor pressure due to a bad regulator (I have two)
Oxidized contact on the over temp sensor
The sensor is right down front after you remove the access panel on the side AWAY from the circuit board and blower.
The way to test for the sensor as an issue is to short the two contacts when the thermostat calls for heat. If it ignites, the sensor is likely the problem.
For me, cycling the connector on a off a few times and then crimping it to make it tighter was all that was needed.
That was 5 years ago.
No problems since...