Sorry for the delay, it seems no others here wanted to take a shot at this concern. Since it occurs while still running just coming to a stop, that rules out the converter drain back valve. Since 1st works once engaged I do not suspect a clutch or piston seal issue in the forward clutch. I believe your going to find a lave or spring or bore in the valve body damaged allowing the pressure to the forward clutch to bleed off or delay in getting to the clutch. Do you know how to remove and overhaul a valve body to check it out or rule it out?
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