My 82 had the same problem a year ago.  When I pulled the handle out, I
found that the pot metal of the handle had cracked, allowing the steel
square shaft to rotate a bit in the handle and permitting the handle to
sag.  The pot metal that the handle is made of isn't up to the job of
decades of pulling on the handle and slamming the door into the pillar.
Keeping the sliding door well lubricated and using the handle just to bring
the door up to speed
 and letting it's momentum do the latching will add to it's longevity.  I
bought a new handle from GoWesty and it works great.

Brett

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Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera