Thanks for the reply,

I'm hoping it is just bad RAM. That at least would be straightforward to replace. Seems like many types of old computers fall victim to bad RAM so it wouldn't be too surprising if this was the case.

Luckily there's an electronics distributor just down the road from me that sells 4164 and 41256 DRAM chips, either of which (I think!) can be used in a Mac 128.

Incidentally, about the RAM for the Mac 128 - Do you happen to know if it is 4164 DRAM or 4264?

I believe I've seen some reference to both and I'm not sure which it actually is. I haven't yet opened a 128 to see for myself, but hope to do this as soon as I can clear a space to work on it!

Thanks again.