Drop pan drain fluid, rinse down the valve body with brake cleaner and replace the filter. Honestly this sounds like worn seals internally which is causing the rear clutch to not have enough pressure to stay engaged from an idle. Check the trans cooler lines, hoses, and flush out the cooler as well. Any restriction there can also cause this. Does it do the same thing in manual 1 ? If it does not do this in manual 1 then likely the overrun clutch which is used to holds the low reverse drum stationary when in Drive mode. Placing the lever in Manual 1 forces the Low/Reverse band to engage and hold the drum so if the overrun clutch isnt holding the problem will go away when placed in Manual 1. If it does this in both modes Drive/Man 1 then likely the rear clutch is worn or not receiving enough pressure to hold proper engagement at idle. Front pump could be worn, general worn seals in the trans etc. Providing the pump is coming up to pressure the trans may last a while longer if your easy on the go pedal. This is of course assuming were speaking 727 here.
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