'99 Chev Malibu, 3.1 v6, 4T40-E four speed auto tranny, 173,000 km
(108,000 miles).

About once every month or two, when shifting from reverse to drive,
there is a delay of a few seconds before forward 'engages'.  This been
happening since the car was new.

No abnormal debris in the pan.  Fluid level is correct.  Fluid &
filter changed regularly.  No codes in the PCM.  No abusive driving
and/or teenagers involved.  

Transmission operates flawlessly, except for this one occasional
'annoyance'.  I'm stumped.  Ideas, gentlemen??