There are a few things that can cause the spindle to drop. I would check with a dial indicator with the machine off and running. The vibration along can cause it to drop. Also, as you mentioned, the motor mount is one but also (2) the backlash in leadscrew assembly, (3) the handwheel shaft bore, (4) , the mesh between the gears, (5) the thread tolerance from the bevel gear to the acme screw and (6) the clearance between the bevel gear and the top plate. I would check all of these and take note to their condition as they all play a part in whether or not the spindle creeps down. I have a replacement leadscrew and bevel gear from Mcmaster. I have yet to cut the acme threads as I need to order a special acme tool. These machines, as great as they are did not have the best covers in place to keep grinding dust from migrating its way to these key components. A little bit of dust goes a long way at chewing it up.