Hi, If might the keyboard. The IIgs keyboard is known for the connections that would break down. I repaired quite a few for the schools. Try a different cable and keyboard to be sure. If it is the keyboard is bad, it is a easy fix. You just have to resoder the connections. One time I had to add a little peice of small copper wire to bridge a broken part of the connection.
It is not the keyboard nor the cable, both works fine on my other IIgs.With the IIgs floating around low $$ , I think I have a IIgs for doing some hardware experiments. As I wrote, he works fine just has no keyboard. So booting and running programs is working, even gaming as long no "press y/n" is there.
Thanks for your advice!