Although the reason most likely resides on your logic board, I would just like to add that I had the same random kind of ADB trouble with my SE/30 in the past. I spent hours in vain diagnosing my logic board. In the end, I said to myself, what else could it be? I then thought about it an realized that whenever a problem occurred it was with my keyboard. I opened up my IIgs keyboard and sure enough there were a couple stupid electrolytics in there that had leaked. I snipped them off, cleaned the board, then replaced them with tantalums, and so far I've had zero ADB troubles.
What this means is that you folks may also need to consider cap replacement in your peripherals too. Don't exclusively think about the logic board!