Its either the 5 opto's on the opto boards behind the targets or its an issue on the opto controller board located under the playfield. Replacing the optos or the boards (there available online) behind the targets is not that bad. No need to remove the expander just lift the playfield. At the bottom of the expander first remove the 3 bank dalek stainless steel target holder by removing the 4 screws (2 on each side). Note you may have to remove the target reset bracket on the right side to get to the 2 screws there. Then remove the 5 screws holding the black plastic 5 bank targets. Unplug its connector below and pull gently through. Once out remove the 4 or 5 screws from each side holding the opto boards.
Note test the targets by hand before you disassemble to determine which optos need replacing.