The game was almost 100% running ok. But here were bars across the screen visible only when background was black. Nothing noticeable in game.
I swapped the A and C boards and narrowed the fault to the B board. Then I swapped the ROMs and PALs and found the B board itself was faulty. I probed the few ICs on the board but didn't find any suspicious one. It's the third time I face this exact same issue on CPS1, two previous times it was simply a cut trace. This time made no exception. I found only one broken trace and patched it with the core of kynar wire.