I have a Goodman furnace that is 12 years old that is having a flame roll out on the first chamber intermittently. I have checked stack for blockage, what should I check next

Check gas pressures. Also check to make sure orifices are clear. Next, a CO test should be done (could have a crack in the heat exchanger,, worst case senerio). You may also have a partially blocked heat exchanger. Flame rollout is a very serious issue, so if you can't find the cause, you should call in a tech. Don't be surprised to find that a new furnace may be suggested.