I can almost asure you that if the car jerks from park to drive or reverse that its the tranny seliniod. The one in my car went bad and it did the same thing you guys are going through. My brother in law is a lincoln mechanic and he told me 9 out of ten times its that.
I get this problem when the engine is cold, when you start the car try giving it like 25 seconds to idle down a little bit, you will actually watch the RPMs drop at this point try putting it in gear and see what happens.
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Exactly what I do.. The engine goes to what 1500 at start? and 20 some seconds later drops below a grand. I still however find the knock when going from R to P.