Car breaking up at top end.

I have a 440 60 over. The car is breaking up and tach bounces around from 6000 to 7000 through the traps. Losing 3 mph. I have replaced plugs and wires, cleaned fuel lines and filter. Checked wires and connections. Fuel pressure is consistant. Using a 1050 Dom. Would my next step be timing? Or maybe a bad MSD box?? Any advice is greatly appriciated. Thanks
Author: moparkaiser