77' Westy sliding door problems

My 77' Bus sliding door likes to fall right off. I will tell you why
it happens, and maybe someone knows what I can do about it. On the
lower track where the 2 rollers glide, there is a dip in the metal
about 1" across, and when the rollers reach that area, they dip into
it and then there is enough clearance for the roller mechanism to come
right out of the track. The front part of the door falls to the
ground, bending the door as it hits the ground of course.
Simultaneously, the roller on top of the door comes out of its track,
causing great strain on the roller mechanism on the rear of the door.
Thats when the door falls right off on top of you. At that point you
are in trouble.. especially if you are by yourself with nobody to help
you try to get back on. Anyone else had this problem? How do I correct
that little dip in the metal on the lower track. It isnt rusted or
anything. This is the 4th bus I have owned, and never had this problem
before. And yes, I have adjusted the roller ball on the top of the
door as high as it will possibly go! Any advice? Thanks!