The problem is the BASIC ROM(s). Either not seated properly or defective. Try re-seating first. Then replace with an EPROM using same revision images in the hardware section of this site as original for your model. If you have the production ROMs your best choice is a Motorola MC68764C or MC68766C EPROM. If you have the EPROM adapters as original then it is very easy, just use a common 2764 EPROM. Nothing fancy, but make sure the part you use either matches or exceeds the originals' access speed. A slower part could cause intermittent timing problems.