It could be a bad ADB connector. I had these on both sides of the IIgs keyboard I used with my SE/30. I resoldered the pads but still had problems. I then replaced the cable but the problems remained. It didn't happen all the time, so I tried to live with it for about a year. But after a while it got to me so I opened the keyboard up and examined the keyboard PCB. Sure enough two of the capacitors on the board were leaking. I replaced them with tantalum versions (which will never leak) and all my keyboard problems are now a thing of the past.
Some people will lead you to believe that only the main logic board (or in some cases, the PSU as well) need new capacitors, but that is not the whole story. If you have electrolytic caps inside your keyboard, you need to swap those out too.

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