This is a common occurance in fruity an even if u up your memory it will still crackle, it just wont crackle as soon as a pc with less memo.
Best advice i can give is dont use so many vst's, and export things that u dont need to constantly have a vst on.
Another tip is go to options>audio> and drag the buffer rate to the right as much as u want, mines on (260ms) but it will differ on your pc, this will make fruity seem slower, but it will last longer before it starts crackling, also turn the priority to highest.
Peace!