> No way you know of to test the circuit board?
Does this 1993 unit have a board? I have sold Ruud for 20 years and I don't think their furnaces with boards show up on the market until about 93/95, you could still have the old adj. fan limit.
- never mind I saw you have a inducer motor.
I have seen furnaces act like this with bad stats or shorted low voltage wiring, you can check to see if it's low voltage short by taking all the low voltage wires loose from the board and see if it still does the same thing.