Furnace will light, but burner stays lite for only 30-40 seconds, then goes off. The fire streams that shoot into the burner are shorter, smaller than previously, when it worked. The repairman has replaced the control (pilot push down and then main burner turn-on), still the same problem. Lights for appx. 30-40 seconds and goes out. He replaced the electronics board, still same result. The blower/fan works as it is supposed to. Pilot also burns as supposed to. Solutions, advice, please.

Sounds like you have a parts changer instead of a service person. Has anyone checked gas pressure?