I posted a few replies ago with my almost identical problem. I had a
professional come in and they located the problem. It was the pressure
switch. So basically the first blower would start...then the pilot
light would ignite...then the main gas would only come on for just a
second...followed my the main fan. The pilot light would remain on and
the furnace would act as if nothing was wrong. Once the pressure
switch was replaced..all was well