it could also be the 74ls244's all the ram is connected to, When i assembled and installed a MacGusto II upgrade board onto one of my 512k boards i got something similar and the install guide pointed to one of the 74ls244's being bad (in my case this turned out not to be the problem, it turned out i had a short under one of the ram ic's) So if replacing the ram doesn't fix it try swapping the 74ls244's