before stripping to check spider assembly check the lock nuts that hold the counter weights in place they can come undone even with lock nuts fitted 
The top weight  is accessed by removing the lid, 3 screws at top rear of the  machine.
The bottom weight is via the rear lower panel 6 screws . a tell tale sign  that the weight is LOOSE is concrete dust on the floor.
                
 wirecutterman
                
 March 2009