1993 vintage furnace would use a 'timed delay relay' where DC power is always on one side.......takes DC from Themostat to make up coil which then allows the constant DC to go to fan motor
So either the 'timed delay relay' is failing or your have a bad thermostat
Is it time for YOUR Medication or Mine?
Dodge 3500 w/Tractor Motor & NUWA 5vr
US NAVY---USS Decatur DDG-31