440T4/4T60 Drops into Neutral at Stops

Hi, I have a 91 Buick Lesabre, 90,000 miles with a 440T4 (4L60) transmission. When driving from a cold start, the transmission will drop into neutral when coming to a stop. It may do this 1 or 2 times within the first 5 miles of driving. Once warmed up the transmission works fine. When it does drop into neutral, it will re-engage within 5-10 seconds. Manually shifting in the 1st will also re-engage the transmission.

I have replaced the vacuum modulator – no change. I have also removed/cleaned the pan, changed the filter and refilled to the proper fluid level – no change. The fluid is clean, not discolored and no burned odor.

Is there anything else I can do to service the transmission in the car or am I looking at a rebuild to correct the problem?

Thanks in advance!

Sorry it needs an overhaul it is loosing internal pressure.