[Brian in Tucson]

Take off the clamshell and lay it on the pavement. Are all four corners contacting the pavement? If not, it's tweaked and needs to be straightened. While the clamshell is off, try cycling the top. Still clunking, you need to synchronize the transmissions. With a little help (an assistant) you can untweak the clamshell. Grab it by all 4 corners and twist it to make all of them touch when you put it on a flat surface.

I'm going thru all this BS, too. Need to get as close to basic as you can. Synchronize the transmissions, and make sure the linkages to the top are both the same length. One side has to precisely mirror the other side!