I have been through this just a few weeks back, the low speed part of the fan motor went bad. I found this out the hard way one night around 8:30 stuck in stopped trafic on I-75 when the PT over heated and I had to have it towed home.
The Black wire is ground, the yellow is High speed, and the Dark Green is low speed.
Good luck, but sounds to me like you need a new fan motor.
