z axis dropping during cut help

I have noticed during heavy cuts, my z seams to drop. It's almost like the slack in my Z is sliding down during the cut. My backlash setting is .018" in my Z. With stock screws is there a way to reduce the backlash in Z? Do yall have any other ideas for me?

G0704 using stock screws.
KL-6050 stepper motor drivers
C-10 parrallel break out board
KL 600-48 48VDC/12.5A Switching CNC Power Supply
nema 23 570 oz (KL23H2100-50-8B)

David Branch