Hi,
Here's a possible clue, though: under the 1.6 kernel (which doesn't exhibit the problem), in systat vmstat the "kbd/ports" counter normally shows nil when the system is idle, with interrupts only when I start typing. On the 5.0.1 kernels, there seems to be a constant background level of 8 interrupts per second on kbd/ports. I thought I also observed this number drop to values around 0-3 when the keyboard got stuck, but I couldn't reproduce that. Useful or red herring?
Could you itemize the hardware you have? I have an A1200, too, with a Blizzard 1260 and phase5 SCSI, but I have neither run an installation kernel for ages, nor have I had this problem.
On the other hand, I use an external keyboard (I physically lifted the soldered pins and soldered wires to an external 5 pin connector; my machine is in a 1U rackmount case), so it could possibly be a problem with the internal keyboard controller.
Another thing to try is to switch to wscons and see if the wskbd0 device behaves differently. Let me know if you need a custom kernel built.
John Klos