I have an older furnace and recently the blower has been running non-stop whether it was heating or not. I found that opening the bottom panel on the furnace and flipping the switch from auto to manual and back would fix the problem for a while but it would start doing the same thing just a little while later and now it goes back to the problem almost immediately. What can I do to fix this? It is a southaire system

Sounds like the fan control/high limit control is bad. Replace with like same.