On my Escort ('93) the first problem I had was a bad connection at the motor. One of the two wires connecting to the motor was burnt. This is caused over a long period by power arcing across a bad connection. That's what this problem sounds like to me. If you have good connections there then it could be the motor is worn out. Typically IF a resistor assembly goes bad it will only affect one motor speed and will not affect hi-speed. Anyway, have a look at those connections.
