Why wont my car go over 30 mph

1997 chev. Blazer ls 4wd w/ 156,000 miles when you get to about 30-50mph it acts like it has no power and bogs way down to the point can't run over 30mph checked cat ok replaced. Ignition module, wires, cap/rotor, pickup module, coil pack cked plugs look good ran scan shows po300 if not misunderstanding means multi misfire at a loss of what else it can be please help

Check and see if you have a good ground from the block to the frame. And from the solenoid to the battery see if the cable is pinched