I did like Dean says, larger grub screws. I also filed flat recess on shafts, where grub screws go. It was quite easy using small square file. It seemed to me it would help. Locktite also. In fact my major mistake on the first build was not using locktite on all screws. Result was from vibrations or whatever at least 2 screws difficult to access
loosened so i wasted some time tightening again all screws on my friends machine. On my current build i use Locktite on all screws and i hope all will be perfect long time after i am finished. Same goes for the supported rails. When all was mounted and square, i carefully poured locktite between screws and rails. later checked it, it worked. Then i fit the plastic caps.