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I would check power to the blower relay. I say this, because when you hit 75 degrees, I'm assuming about the "desired or satisfied temperature", I would think that maybe the blower relay maybe sticking? might this be the case? If you can give me more infor, maybe we could figure it out together.
to be very honest with you,, I have no clue! Based on what you are describing, I would suspect friction at start up or shut down of the blower. your telling me you just replaced the blower motor, so, I'm thinking maybe it is not aligned properly in the blower cage? I have never heard of anything like this, but would like to hear the result. I will open this up for other experts to see if they have any answers. Sorry I couldn't answer your question.