Well I had a similar problem with my A2K, whenever you pressed button one of any device attached to the 2nd user port, the machine would freeze and upon reboot, I would get all kinds of funny screens, if I shut it down again, all would be fine until you pushed button one again. Mind you I did also notice some oddities with the keyboard during normal operation as well. Once I replaced both CIA's the machine was perfect. Not sure how you would fare replacing the CIA's on your A4KT as they are surface mounted, might be a tad costly, so I would examine every other possibility first.
