problem solved on this machine.
I've seen this question ask in the past, but never a solution. This machine was a unused one and Mocha. When I removed the large screw I could see something black, but did remove the extension / bed Today the bed came off, I found that the black is paint. The machine was painted black then painted mocha. my guess is the black machine don't have this problem.
you see the type of screw , where the threaded portion is much smaller than the shaft. in order for this to work that screw must bottom out at the step/shaft , then the concave washer and the flat washer work as a spring ..
with two thicknesses of paint on the extension casting . the screw wasn't able to bottom out ( tighten) against the machine body and would pivot each time the extension was raised and lowered. loosen the screw each time = floppy extension
I removed the paint, which then allowed the step on the screw to stick out just a little, and to hold tight. yet working the extension felt a bit stiff. I used a fine flat file and removed a little of the aluminum. this showed an indent at the screw hole. I took the aluminum down , so that the indent was gone .
out come is it now works perfectly. by all means oil the screw pivots