[Retro]

Initial thoughts: sail switch? bad connections? low voltage? Does the fan speed seem to vary or just the flame? Any chance some carbon got down into the burner while you were cleaning it?

It stays running, just sounds like someone is in the next room on a punching bag. No back fire just snuffs and relights itself right away.
I used a hacksaw blade to clean the burner then blew it all out. 
It almost like its starving for fuel. The other two furnaces are working fine. 
Possibly some carbon in the fuel feed to the burner?