New to list- great so far!

My 90 Carat door locks do the following:
Starting from completely locked position, key opens either driver or
passenger door.  Opposite door and rear hatch locks open as prescribed,
but sliding door lock does not.

Starting engine does not change condition, but putting into drive and
moving about 5 feet results in slider lock opening!  Also, the locking
cycle works properly: i.e., locking either front door with key results
in all doors locking.  I have shaken door handle, cleaned contacts on
slider door that meet at front-- no avail.  Driving me nuts.  

Thanks for any help in advance.

Ed
88 Vanagon Wolfsburg, dead, buried, gone......
90 Carat 105K