Assuming the coprocessor board is OK, this is most likely either a downlevel PLA (only 906114-05 or 906114-06 will host the 8088) or weak power supply. If you test the power supply do not forget it is a switcher so do not test without a load! Also can be caused by installing an 8087 math coprocessor into the coprocessor board socket for same. This was originally designed to be supported but there is a hardware flaw in the prototype design and it will cause a hang - do not install a math coprocessor.