Trane xv80 starts and runs fine... there is nice blue flame, everything is blowing, then the main blower quits, flame keeps going until safety device shuts it off. This is all with fan in auto or on position

Without call for heat, fan runs all day in on position, and does not run in auto position.

What could be causing this?

Probably a bad circuit board. Check for evidence of burned circuit paths or relays. You may be able to wire in the blower hot for interim heating.

Could also be a defective motor. You might try to feel the motor with the power off to see if it is over heating.