My tempstar sp80, maybe 15 years old, blasts with flame a few times before running. What's going on?

My tempstar sp80, maybe 15 years old, flames out 2-3 flame burps before fireing up! Does anyone know why?

I think you need to find the flame sensor and clean it.

It is a wire that sticks out into one of the burner flames. It is attached to the frame of the furnace with one screw and has a ceramic base. So look in the flame area of each burner till you find it. When you do take out the screw, clean the wire with a fine sandpaper and replace. If this does not solve your problem post again and we will try something else.

Before my tempstar sp80 began acting up, the electric ignition would light the pilot, then the main gas would turn on, and all was well. Now the electric ignition and the main gas turns on at the same time, filling the exchanger with gas before the pilot has lit. This results if a violent lighting that blows fire all through the heater, the unit resets, and same thing again. It is rather unnerving. I have talked to folks who have said it may be the flame sensor, and some say it is the smartvalve.
I found the flame sensor at the ignition assembly and cleaned it up. The problem still remained, so I cut the wire to the flame sensor to see if there was any difference. The first couple of startups were different, the pilot lit, and the electric ignitor remained lit as expected and no main burners. The third time the main burners lit violently like before. I'm still scratching my head as it seems inconsistent. I don't want to get a new valve unless I have to. Any other troubleshooting ideas?