taurus transaxle

when driving in overdrive mode and coming to a stop or even just
below about 5 mph , the transaxle drops into neutral and must be
recycled thru neutral position of selector and back to drive in order
to reengage into a drive gear... othewise runs great...1997 taurus
with ax4n transaxle.....does not do this if driving in other than overdrive
position of selector.. what could be the cause?? is it possible old fluid
could be cause?? [email protected]