And what faulty chips on motherboard might cause any of these you mention (address/data/ctrl bus problem or ROMDIS forced high (no access to ROM), or WAITn forced low (Z80 waiting forever)?
Up until now i have replaced almost any major chip (Z80,6845,40010,HAL16L8,74LS244,74LS373,1st bank ram) on motherboard, so is there a chance that some of the remaining chips (for example a faulty 74lsxxx) which i haven't replaced, could cause a so serious problem?